So, for a few days I could not get the iSight to work again on my 400MHz B&W G3. I was baffled for days. Anyway, here is a simple solution I came up with.
With the machine "Cold". Make sure the iSight is disconnected.
Push the Power Button and wait about 10-15 seconds until you hear the OS begin to load. Once you hear it beginning to load, connect the iSight to the FireWire port on your machine.
The Green light on your camera should come on, and then go off.
Once your machine is UP, simply launch iChat.
When you RESTART your box, you should not have to disconnect your camera, but if you SHUTDOWN the machine, you will have to disconnect & reconnect the iSight to get it to work on machines below Apples 700MHz threshold.
My Machine Specs:
400MHz Blue & White G3