I called Apple once, on behalf of a friend, about the same intermittent issues your iSight is experiencing. According to the tech, what's happening to your iSight is a symptom of something wrong with the hardware.
Apple recommends that you reset the System Management Controller (SMC). If that fails to clear up the problem, then your iSight is faulty and you should send it back. In my friend's case, SMC reset didn't fix it and it was later determined to be a faulty cable connecting iSight to the motherboard and was replaced by Apple.
WARNING: Don't use the isight-disabler as suggested by another answer. It's used for preventing the use of iSight (usually) within corporate environments. It might momentarily fix your problem, but the underlying cause remains and the longer you leave the actual underlying problem unsolved, chances are your Mac might go out of warranty and you'll be sorry.
Reference: Intel-based Macs: Resetting the System Management Controller (SMC)