If you're having trouble with your built-in iSight in Photo Booth, iMovie HD '06 (or both), here's one possible fix. If there is a picture of a camera in Photo Booth where the video/pictures come up (or the black screen) with a cross over it, just quit Photo Booth and open it back up again.

This also goes for iMovie '06, but in that app, there's a blue screen and it says that there is no outside camera attached, or the outside camera is not working properly. The only problem with this message is that there really isn't an outside camera attached to the Mac you are working with! However, quitting and restarting iMovie should fix the problem.