As far as I can tell, there is no need for a Thunderbolt webcam. One, you already have integrated iSight/FaceTime cameras/whatever they're calling it these days. Two, if you're wondering about getting a RAW HD stream, USB 2.0 can easily handle that bandwidth.
As spider3 said, I imagine we'll see breakout boxes that have external USB (3.0?), Firewire 400/800, video outputs and maybe even eSATA. Will it come from Apple? Doubt it. They want you to buy the very nice but very expensive Thunderbolt Display.