Move the clock forward to a few weeks ago. I got a new 80GB 2.5 drive in a housing with FireWire and USB. The drive worked off the FireWire bus on my iBook, Mac mini, and the Intel. It refused to spin up on the iMac. Now, how is this related to the iSight failure? It was a clue that the problem was the FireWire bus itself.
I bought a powered Belkin firewire hub for 30 bucks and bingo! The hard drive now worked as it should. It crossed my mind that the iSight draws a fair amount of current, and that this might fix it also. Sure enough, plug the iSight into the powered hub, and it now works just like all it does on all the other Macs. I have a DVD burner (Plextor) that worked, but was sometimes a bit shaky for no apparent reason. It too is rock solid now.
So if you are having FireWire issues, it may be worth 30 bucks to try a powered FireWire hub to see if that "fixes" the issue. What I don't know if this is a problem with many iMacs or just mine, but in reading how many iSight failures there are, I suspect that it is bit more widespread than just my iMac.

