I have a MBP running Linux, following a PowerBook running Linux, and would now recommend against Mac hardware.
Apple have a nasty habit of changing out parts in the middle of a production run. That is fine if you are running OS X, but very bad if you end up with a part with no Linux drivers. I ended up with a DVD drive that can't even read data from out of region disks and a wireless card revision with flaky support when other people who bought the same model at the same time have no problems.
The M1330 laptops from Dell look pretty nice, are compact (13" screen) and are very good value. I tried one out a few weeks ago and liked the screen. Plus they come with vendor support for Ubuntu and it's not a bad idea to vote with your wallet to encourage that kind of thing :)