I wrote this back in 2006-07-15 and the guide is also available here, but a lot of people still contact me with miscellaneous tips and tricks. Since I don't have the time to update the original post with these it's better if they can comment directly on the post - hence the blog post. BTW, my camera is still running strong three years later!
I don't know anything about other camera models and to be frank, all I know about this one is written below. If you have a different camera or a different problem, try asking at some forum. This one is pretty good: http://www.dpreview.com/forums/
Disclaimer: I take no responsibility for what might happen if you follow these steps. Your warranty will be shoot and if you screw up, you're to blame. This is just a documentation of what worked for me.
It's not uncommon that the sub-mirror assembly on Canon EOS 300D / Rebel brakes after everything from 10.000 to 50.000 shots (I got 23.000 out of mine). This is due to a plastic pin snapping (not visible from the outside, even with the lens off the camera). It is possible to send the camera to Canon for repair. The cost is usually something like US $200. Considering what a new camera body costs this is not a cheap option for an old camera. If you're camera is still under warranty Canon should repair it for free since it's a design fault on their part.
How to diagnose the problem
All of your shots look like this (note: in some flash/sync combinations a similar phenomena can occur):
This is due to the sub-mirror blocking the CCD, causing a dark area on the photos. Removing the lens and lifting the mirror carefully by hand, one can note that the sub-mirror wont close into the main mirror. The sub-mirror is used by the auto focus and should fold into the main mirror when it lifts out of the way.
The sub-mirror can be flipped into the main mirror by hand
After doing that you can use the camera but the auto focus wont work (setting the focus manually works fine and the camera works just like normal in manual mode).
Fixing the problem
The camera can by fixed. It's not an easy operation and requires some skills with small screws, patience, even more patience and some good luck. But what do you have to loose, the cameras busted anyway (unless you feel you can live without auto focus, in that case, just glue the mirrors together and live with it).
Here are the tools needed for the job:
Needle, glue, small flat screwdriver, long thin small Phillips screwdriver (not like the one pictured), paper clip, magnet, tweezers (for aligning screws), wire cutter and needle nose pliers (last two items not pictured)
You also need good discipline when it comes to taking the camera apart, placing the screws in an order you can later reverse. Don't forget witch screw goes where, they are of different length and threading. There are close to 30 screws in total.
Let's begin with the disassembly
Note: All left/right-directions are made on the assumption that you hold the camera just like during normal operation, facing away from you. That is, looking at the back of the camera.
1) Start the camera and make the flash unit pop-up. Do not close it again during the disassembly, if you do you wont be able to remove the front cover of the camera (trust me, I found out the hard way).
2) Remove the lens, the viewfinder piece, CF-card, main battery and back-up battery. If you have the shoulder strap mounted remove that too.
3) Turning to the left side of the camera, remove the screws pictured. Note that the top screw is longer than the bottom one.
4) On the back of the camera, carefully pry the screw cover loose with the needle. Then unscrew the screw behind it.
5) Turning to the bottom of the camera, unscrew the three identical screws marked with arrows.
6) Under the lid for the CF-card, locate and unscrew the two identical screws. These are slightly longer.
7) Now, carefully remove the back cover from the rest of the camera. There is a flat cable going from the black and gray information LCD-panel, lift the lock on the connector and take this cable out from the circuit board. The picture shows the connector unlocked. Just lift the black/brown part with a small screwdriver and then pull the cable straight out.
8) On my camera, the silver rim with the "Sony"-marking around the large color LCD came loose and stuck with the back cover of the camera. If this happens for you too, repair that part assembly before you continue. From bottom up it should be the white backlight, a black plastic rim, the LCD itself and lastly the silver rim.
9) Now it's time to remove the front cover. Turn to the right side of the camera and remove the three screws. They're all the same size.
10) Turning to the bottom, remove the four screws. The single one on the front is longer then the three others, who are identical.
11) Now unscrew the two screws on the front right under the flash. These two are slightly longer and identical.
12) Now look inside the battery compartment. Towards the front there are two screws (identical). These little buggers took me some time to locate. With a long thin phillips screwdriver inserted from the back you can unscrew these. My screwdriver was a little to thick but some mild bending got it where it needed to be.
13) Now slide the hinge on the battery hatch away and lift the hatch off. Cool feature! :)
14) Unscrew the hinge base. Note that there is a difference between the left and right side (different spacing between the holes and edges) so pay attention when reassembling this one (you can't go wrong). The screws are identical.
15) Now all screws holding the front cover are removed. With gentle force, lift the bottom away from the tripod mounting hole and lift the front cover off the camera. No cables are attached between the cover and the rest of the camera. Thankfully!
16) Okay, that was the easy part. Now let's move the top cover a bit from the camera. This ones attached with quite a few wires and flat cables but no soldering required! Start with removing the two flat cables from the top left corner, looking at the back of the camera. Just lift the brown hinges and pull the cables out. Make sure both of these come back to the outside when putting the camera back together. One of them escaped inside the top cover for me costing me some additional fiddling.
17) Remove the long screw on the top right.
18) Remove the two (identical) screws next to the view finder and the dioptri screw too (unscrews just like a normal screw).
19) Locate the black tape on the right front side of the camera and pull it loose at the top. You can leave it stuck on the bottom if you want. This is just to give the wires more room to move around as we lift the top cover off the camera.
20) Carefully dislodge the top cover straight up. You cant remove it fully but you can move it a few mm upwards. Then unscrew the two identical mini-screws holding the shoulder strap mount on the left side and then remove the shoulder strap mount.
21) Find the two identical screws that hold the circuit board on the front of the left side of the camera. One is quite obvious (not pictured) and the other is hiding a bit. Also note that there are two pins, one next to each screw. These help holding the circuit board in place but when moving the circuit board you need to work with them.
22) Now you need to move the circuit board away from the camera. Moving it towards the top and at the same time moving the top cover away is the way to go. Be really careful here, the big capacitor holds something like 220 volts (measured) even though the batteries have been away from the camera for quite some time. Try not to short-circuit anything with that voltage! You might loose a circuit board here so pay attention to what you are doing. To make matters worse it's a really hard task to move the circuit board out of the way. On the picture below I have marked the three main problem areas. Moving the circuit board into this location is the best way to move it out of the way. I almost went mad during this step.
23) Congratulations! You have now successfully located the problem zone and can actually start the repair of your camera! The culprit sits where the green arrow indicates.
24) Taking a closer look, this is what you find. This is the pin. There is no obvious way to remove the broken pin from the mirror housing.
25) With the needle, trace the ring around the pin. This loosens up the glue that attaches the pin to the mirror housing. After that, carefully pry the pin away from the mirror housing using a small flat head screwdriver. Someone actually managed to come this far only to drive the screwdriver into the mirror housing totally wrecking the camera. Learn from him/her.
26) After some careful bending, the pin gets loose from the mirror housing. Note that the glue is quite gooey and not of the hard kind.
27) Here you can see the broken "pin". The actual pin has broken away from the base and is nowhere to be found. When Canon repairs the camera they replace this part with a metal one. Let's do the same, shall we?
28) Straighten a normal metal office paper clip and insert it into the hole in the mirror housing. Let the mirror be in it's normal down position. Hold the pin and try flipping the mirror up and down with your finger. Note that the sub-mirror now moves with the main mirror. That's the ticket. When you find a optimal amount of paper clip penetration, take a marker pen and draw on the paper clip. This way you know how long the protruding pin needs to be.
29) Now start bending the clip into something with a base and a protruding pin, there is no need to worry about clearance towards the circuit board - lot's of space there. When you feel satisfied, cut the clip down and try it in the camera. The mirrors should move around freely and the sub-mirror fold into and out of the main mirror.
30) Place the new pin into the mirror housing and put a drop of some thick glue onto it (I used Epoxy resin, a glue gun might work too). Make sure the clip does not float in the glue, it should be flat to the mirror housing. Just like when you confirmed it's operation. If you use a loose/running glue it might find it's way down to the mirrors, not good.
31) Check that the mirrors are moving so that you did not make the clip to long or short and that no glue found it's way to the mirror hinges. I got lucky! :) When all seems to be working, it's time to reassemble the camera. Just start scrolling back on this page step by step. When remounting the back cover, make sure the sensor for the CF-card bay door is free to move. Not focused picture below. After the full assembly the camera should work just like normal again. It worked first try for me but you might need to go back and re-seat some of the flat cables.