I have a MacBook Pro hooked up to an Apple Cinema Display. It is *very* common for the external display's camera to be unavailable to my apps.
Since I do not reboot often, it seems very possible that an app (or its background process!) that is not actively using the camera is still locking it.
Figuring out what's using it (if anything) seems like the first step in determining why I can't use the camera I want, when I want it.