I have 2.6.8 on my PowerBook G4 and it works fine so probably it's a different problem.Looks like 31r1 includes 2.6.8.
(I also have Ubuntu 5.04 with kernel 2.6.10 and it works fine too)I'm not an expert but you may try to boot from your cd and, when asked, digit install video=ofonly.
I was also adviced to try: install video=nvidiafb:off install video=offb install video=rivafbThis had no luck with me but this may help you to understand if it's a problem of the kernel or your computer.
HTH Marco.