With an Intel iMac running Leopard, I kept getting an error that the built-in iSight was in use. To fix this problem, I now do this from Terminal as an admin user, and I can again use the camera:
$ sudo killall VDCAssistant
I do not even have to restart iChat for the fix to take effect. The way I deduced this is that I ran sudo lsof | less -i, searched for usb, and found this VDCAssistant process using some VDC (whatever that is) library that the QuickTime component of iChat was using, too. So I decided to kill that process.
The system respawns the VDCAssistant process, but after that, the iSight camera can be used again in iChat -- at least until it suddenly becomes unusable again, for no particular reason I can determine.
