Booting PB 12 from ubuntu cd

[trive at poc.it]

Hi guys,
first of all: Ubuntu is great!
I installed it on an iBook and found it just wonderful.

I've just bought an Apple PowerBook 12 (G4 1.5ghz, 512mb ram) but I have a big problem: it doesn't boot!
Actually it starts booting but it freezes on
"MMU mapin
MMU setio
MMU exit"

... I don't know what to do! I've found on the internet a modified version of the kernel but how do I updtade the kernel in the cd?
I've also tried with Debian etch testing and the powerbook freezes at the same point.

Hope to hear from you soon,
thank you so much,

Mark.