I am having the same problem with a 17" iMac intel core duo.
after upgrading to 10.4.10 (via a 10.4.10 combo update downloaded from work because internet has not been set up at my new apartment yet) my built in isight fades to solid black within 2 seconds of being put to use in any applicaion... this is very frustrating and i would like to find a solution if anyone knows of one...
Also i have one of the old firewire isights from another computer so i tried plugging it into the iMac that is having problems and both ichat and quicktime work without any problems as long as they are set to use the external isight camera...
if anyone knows how to fix this problem please let me know.
thanks,
jason