I can vouch for ubuntu; I run it on my powerbook for live shows. I'm still tied to os x in the "studio" *cough* because of ableton and various vst's etc...
one thing with ubuntu, is that I'm not sure if they offer a low-latency kernel for ppc, so you might have to jump through some hoops and read some forums if you wanna set one up. I never bothered, because the stuff I'm doing is mostly feedback processing anyway; not really dependant on latency ; p
that aside, ubuntu seems to be the most painless to setup. it autodetected most of my hardware, and there's a killer faq on the ubuntu forums for setting up wireless etc. for pd you can use the extended installer that hans puts together http://at.or.at/hans/pd/installers.html. this works really well for me.
one very definite plus from using linux is you can run pd on blackbox and turn everything else off. no dropshadows, no itunes daemons, nothing but pd and a few bare minimum system processes. this is very cool if your powerbook is old and cranky like mine. I can load all my patches at once and it only hits the cpu about 30-40%!