Hardware:

If your graphics card supports two monitors, or a TV out, just plug in a dummy device. This usually does not work for a monitor unless you actually plug in a monitor, but the TV out can be easily tricked into thinking a TV is there...


Software


For Windows:

http://www.zoneos.com/zonescreen.htm - Virtual Monitor

It takes away the Vista effects, so...


For Linux:

x11 monitor forwarding


Comments:

I did this with my T3 some time ago. Obviously, over a bluetooth link with a Palm device, things were slow, but don't expect video or something. Also, the DPI difference makes thing complicated. Something big on the screen will be TINY on the tablet. Just FYI....