I saw these instructions today on MacBidouille, a French news site. Translation is as follows:
- Download the macam (free) or IOExperts (not free) drivers for your USB webcam.
- Test that you can get the webcam feed with another application (such as video viewer).
- Download and install Unsanity's Application Enhancer, if you don't already have it (you will need to relogin or reboot).
- Download and install the iChatUSBCam application enhancer module. Be sure to shut down iChat first.
- Start up iChat, and the camera icon should appear.

