I'm using a firewire iBot - got my clearanced from my local Micro Center - it had been returned by someone. Works great - haven't had any problems with it. BTW, I recommend EvoCam (http://www.evological.com/) for a webcam app.
My cam is up and running off my iBook G3 500 right now as a matter of fact ... http://www.oneblueshoe.net/obscam.html if you want to check it out. The lights are off at home, so the room is fairly dark.
My favorite cam has always been the 3com Homeconnect USB for my PC, but it doesn't work on X, and they've stopped making them. I heard a rumor that another company licensed or bought the technology from vicam and was going to come out with a new model (hopefully ieee1394), but can't find anything public on it. Anyone have any more info on this rumor?
Good luck!
Michael.
If you haven't bought a webcam yet, there's a company that just released a driver that will make a lot of PC usb web cams work in OS X. It's only $30.00, and you can try it out for 30 mins at a time as shareware.