By using these instructions How to Install Ubuntu on Android! I have managed to install a chrooted Ubuntu on my SGS2. I had to make quite a few changes to the procedure (I'll write them up if anyone is interested) but it wasn't that hard. You would have to know Linux pretty well to debug the above instructions on the SGS2 though. I can now ssh into my Ubuntu installation from any other machine on my network including, of course, the SGS2 itself. Tomorrow I shall try to install a GUI. That's a little ugly, requiring VNC, but it looks simple enough. Perhaps someone will write an Android X server. Perhaps they already have, I shall have to investigate further.