I own a ThinkPad X61 Tablet.
X61 Tablet
ThinkWiki had a very excellent set of pages about ThinkPads. Installation was smooth and no proprietary drivers were needed.
I initially installed with Jaunty; I have since done a clean reinstall with Quantal on this machine. I also referred to Luke's page on a slightly different tablet for tips.
I have a home-cooked setup of scripts (partially stolen from Luke) for screen
rotation with the accelerometer. The modified hdaps_ec kernel
module that you need can be setup with m-a a-i tp-smapi.