I've already tried the following actions, without any success:
See if the issue is application-specific. Try another application (iSight works with applications like iChat, PhotoBooth, and iMovie HD 6) to see if the iSight camera exhibits the same behavior in all applications. If it only happens in one application, try reinstalling that application.
See if the issue is user-specific. Test your iSight camera in another user account. If the issue only occurs in one user, the issue would be isolated to user settings.
Find out if the computer recognizes the iSight Check System Profiler (in the Utilities folder, inside the Applications folder). Under the USB header, check to see if the iSight camera is detected.