The drivers for the more popular brands of tablets have been worked out on Linux... but the lesser known brands do not.
When I hook up my Genius Mousepen... It is no more than a fancy mouse, and when I use it... I have to go back and alter each stroke manually, which becomes a very arduous and task. That is why I think this project has so much potential... hopefully to be able to detect any pressure sensitive device and utilize it's features.
I still have yet to try out my tablet on this app.. maybe I will get some time later tonight.