Are you running VM Fusion or Parallels?
I had a similar problem after installing VM Fusion. But then I "disconnected" the iSight camera from the VM fusion and then I restarted my Macbook and then the iSight was working again.
Not sure what the problem was. But I do have to make sure to disconnect my iSight from the VM Fusion. Acually, I don't even use the iSight from my Fusion Window. No problems since.