Can i pipe up about Apple here?
I've just got an iBook, having never used a Mac before and am very impressed with OS X. Granted it's not Linux (although Darwin is Open Source), but it can run pretty much any Linux app you care to throw at it, plus if you really want you can install Linux on them, which apparently works quite well, though I haven't tried it myself yet.
It's very easy to get at the BSD layer underneath the gui, in fact i spend as much time in the console in the Mac as I do on my desktop (running Debian).
They may be a little more expensive than an x86 laptop, but they're well worth it imo.
Andrew