I've toyed with X11vnc before, which simply forwards your X session on a Linux machine to a VNC client (i.e. the tablet); To get your own virtual display/etc it's possible to muck around with your x server settings to get a session that stretches across two sessions (I forget what it's called now, but it's what you do when you have two displays that you want to act as one giant desktop), but have not played with that. My biggest problem with it was the speed... on the tablet, even with a customized lower-resolution X-server to VNC into, the performance is still pretty terrible... good enough to use as a VNC client if you're expecting that typical "VNC latency" sort of thing, but definately NOT good enough to use as a "second monitor" that you'd actually interact with on a regular basis... that's just my experiences though...