Interestingly enough, I have a new 3.06 ATI and my camera also doens't register in ichat sometimes. It'll say no camera connected to the computer.
If i quit and relaunch a few times it will reconnect.
I'd also try to do a SMC reset, but unplugging everything from your imac, specifically the power cord and waiting about 30 secs. Then plug it back in.
I did this once when my bluetooth module wasn't being recognized.
HHM