Just got Android running off my HTC Touch HD via the SD card thanks to XDA Developers.
[Project] [15-Nov-2009] Linux Android on Blackstone by #htc-linux team - xda-developers
There are versions for other phones on their forums, so have a look around. It runs off the SD card so is non-invasive and won't destroy your phone or data (back it up first though!). So far it's pretty sweet, and I'll be having a good play over the Xmas hols.