This is in principle possible using xdmx (distributed multihead X) which allows you to create a single desktop using two X-servers running on separate machines.
three scenarios are possible in principle, but none are as seamless as iDisplay, because they all require restarting your X-session at least. I have not been able to get either to work perfectly, but I am running Ubuntu 10.10 and can't upgrade for various reasons. The three are:
1: run an X-server on android (there are two available now in the app store) and use xdmx to combine with your desktop or laptop display. - didn't work for me because xdmx crashed when the pointer moved to the tablet part of the desktop.
2: run a second X-server with vnc backend on your computer, use xdmx to combine that into one desktop with your computer screen, then look at the virtual part with a vnc viewer on the tablet - didn't work for me because xdmx requires all x-servers to have the same color visuals, which is not the case for the vncserver and the real display, and I wasn't able to convince vncserver to change.
3: run two vncservers, one for each screen, then connect them with xdmx and look at each part with a vncviewer on the respective machine. - This came closest to working for me, unfortunately inpout was messed up. it was also quite slow in true-color over wifi. I used this script to start xdmx and the vncs:
vncserver :2 -geometry 1024x768 -depth 24 && \
vncserver :3 -geometry 1920x1120 -depth 24 && \
startx -- \
/usr/bin/X11/Xdmx :1 \
-input :2 \
-display :2 \
-display :3 \
vncserver -kill :2
vncserver -kill :3