Just a note to agree with the recommendation for the T23 Thinkpad. Got one from a seller in Ipswich via ebay a couple of years ago and it still serves me very well - very good value for money.
Just about everything works in linux including sound and ethernet - out of the box on a stock mandriva 2006 install and no doubt just as easy on other distros.
All those handy function keys for volume, screen brightness, switching to external monitor, suspend to ram etc all work although 'hibernate' to disk using acpi doesn't work for me yet. I use a wireless card in the card-bus slot and it switches cleanly and automatically from ethernet to wireless and back again. Using the GUI tools to setup ethernet and wireless networking was very simple - has to be as I'm no techie.
Not a 'desktop-replacement' and no 3D graphics in the X driver but in general it does everything that you need from a laptop on a day-to-day basis. Handles KDE, OpenOffice.org, Firefox etc with no problems. I added a DVD-ROM/CD-writer and K3b works fine.
I take it to school sometimes and connect to the overhead projector and sound system to play music DVDs to the kids as a special treat - helps me to enjoy work more too :-)
Syd