interesting. I searched for "software testing tablet" and I'm not finding much either.
My short answer is it is basically a windows machine with a new input dimension of touch. So test like regular windows software, but consider the alternate way of doing things. This isn't that much different than keyboard shortcuts vs. mouse for menus.
There may be a few usability issues with the touch; I'd suggest testing round 1 without a mouse, just by touch, then move to the mouse, then keyboard, in additional rounds.