check this out:
Installing Ubuntu 8.04 (Hardy Heron) and Ubuntu 8.10 (Intrepid Ibex) on a ThinkPad X200 - ThinkWiki
i'm not saying it'll work, mind you. i'm just saying check it out.
also, keep in mind that while BT4 is based on Ubuntu... Ubuntu it ain't. If this puts you into a no-boot scenario...it's just another friggin' growth opportunity.
good luck with that!