tablet (input device) recommends

I just got permission to order a tablet (input device rather than tablet pc) for the lab. This will hopefully mean that my undergrads aren't going to abduct my tablet pc to work on their posters... or at least not all the time. (Everyone has been wuick to catch on to the graphics manipulation advantages of Kendrick and my tablets.)

Anyhow, I know darn near nothing about tablet input devices. Recommendations? Warnings?

Ideally it should connect by USB, and be usable on Mac, Linux, and Windows boxes (probably in that order - K and I do graphics on linux, but I don't think anyone else does yet... and the lab has more Macs than anything else.)