i've been running ubuntu 5.04 on my T42 for about the past 3 months, and never had an issue.
here at RPI, many students choose to dual-boot linux on their laptops, and ubuntu is definately a great choice. I'm looking forward to trying out 5.10 when the time comes.
If you want to get all those bells and whistles going, and get a couple tweaks in... this site does absolute wonders:
I'm sure you'll love being able to get all those little things working, let me know what you think :P
Just a warning, be careful when upgrading powernow, i had issues with it