If you get an error when using iChat, or any other application, about the iSight camera being in use already, here's a quick hack to repair the problem.

Open a terminal window and enter the following:

sudo killall VDCAssistant

then hit the Enter key. You'll be asked to confirm your password. Doing this will kill the process that is keeping the camera from being used. The process will start itself, and it'll then be well behaved, at least for a while. This is not a long-term fix for the problem, you may still get these errors. But when you get the error, you can repeat this command and the iSight camera will then be available for use again.