A number of people are having trouble with video chats since the introduction of Tiger and iChat 3.0. If you have more than one computer (locally or remotely), one helpful tip to troubleshooting is to allow one of those computers to automatically accept a video chat when invited.
You can turn on this hidden feature from the Terminal by typing:
defaults write com.apple.ichat AutoAcceptVCInvitations 1
To undo this, repeat the command and replace the 1 with a 0 (zero). I'm sure there are other uses for this, e.g. such as using your iSight as a security camera.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050512152831112