Well it turns out I didn't really like Ubuntu on the PowerBook for much the same reasons that it's been annoying me on my workstation - Unity is simply too buggy. I'm now running Debian Squeeze (6.0) on the laptop and I'm really happy so far - pretty much everything works out of the box with just a few tweaks to some configs needed to sort out the remaining issues. The biggest problem is the lack of suspend/resume (due to the graphics card used by the PowerBook - nVidia refuse to release some important specs so the Xorg guys can't get sleep states working) but other than that it all works amazingly well, actually better than Ubuntu did on first install! Even Flash videos (like youtube etc) work out of the box with the latest Gnash player, and all keyboard hot keys (like volume, screen brightness etc) work without set up.
So now my only OS X machine is the Mac Mini attached to my TV - unless I can steal my girlfriend's MacBook every now and then, or better yet grab one of the new Airs :-)