I would think that the linux version of Arduino would probably run fine on ppc linux. But you'd have to compile it yourself, along with all the avr tools.
Arduino 0011 worked fine on my PPC Mac running 10.3.9; I only recently upgraded to an Intel-based system. I COULD do some experiments, I suppose. Somewhere I have a G3 powerbook with 10.3 (?) on it, and my wife still has a G4 powerbook (last of the breed...)