[Alternate Title: Fixing iChat when Video Breaks]
The Setup: My Dual 2GHZ G5, Master, running OSX 10.4.2. Upon installing the latest point revisions of Quicktime and iTunes, iChat 3.1 is no longer able to use my iSight for video, even though the microphone on it works just fine. I can't initiate video chats and nobody can initiate a video chat with me. This applies to both 1 or 2 way chats. An upgrade to 10.4.3 didn't fix this problem, and it didn't bring me a pony, either.
The Fix: Insert my Tiger CD and reinstall iChat 3.0 overtop iChat 3.1. Instructions Here. Delete all my iChat preference files. Install or re-install the 10.4.3 Combo Update. The Combo Update is a fat-bottomed gal -- over 100MB -- so you should probably have coffee ready. Reboot. Re-configure iChat with your preferences. Find that hot userpic you use that looks nothing like you really do and put it back into your profile.
The End Result: You should be fixed. Err, you should have working video in iChat again. Maybe you should be fixed, too, but that's not my call to make.
A side note: I have no conclusive proof that it was the installs of Quicktime or iTunes that broke iChat video for me, but it's the only logical conclusion I could come to.