Quartz Composer use of video capture device, as rendered with QuickTime plug-in:

If you see a black area above either your 'cam is not properly connected, or you may have more than one window trying to use it.

Our thanks to Chris Adamson and his O'Reilly article "Quartz Composer iSight Prank"

More about this as a possible security problem in his follow-up O'Reilly article "Explaining the Quartz Composer / QuickTime for Java security hole"