I own a Thinkpad a20p and I've decided to use it to test operating systems. I managed to quadruple-boot FreeBSD, NetBSD, OpenBSD. and Debian. Essentially I loaded FreeBSD 5.1 RELEASE, then OpenBSD 3.3, then NetBSD 1.6.1, and finally Debian 3.0. I put OpenBSD first on its own primary partition/slice, followed by FreeBSD and NetBSD. I installed Debian in an extended partition with various logical partitions for /, swap, /var, /home, and /tmp. Thankfully I made a boot floppy when Debian asked, and used it to boot Debian and install lilo in the master boot record. I tried to use the advice in this post but found the various installs seemed to step on each other.
While installing I perused various reference materials. Darren Evans' OpenBSD 3.3 Build Guide, Matthew Charles' OpenBSD: Updating source code and upgrading, Gan Starling's What Worked for Me on NetBSD, Keith Parkansky's About Debian, and Daniel Roethlisberger's FreeBSD on the IBM Thinkpad A20p are helpful. This post let me know NetBSD wouldn't cooperate when I tried to list the packages it was installing on the Thinkpad. In other words, the install would seg fault unless I hid the names of files it installed! NetBSD's installer found the wrong disk geometry but adjusted when I told it to check the geometry.