I just tried the Ubuntu Hoary Live CD on my iBook and it worked pretty well. It boots up just fine and includes a nice Gnome desktop. I had already tried Warty on my Athlon XP, but Hoary feels even better and has less rough edges.
Still, there were some things that didn’t work that well: I think the sound is quieter and a bit more noisy than on OS X, probably the driver is not yet perfect. Airport Extreme is still not supported (Not really their fault, Broadcom should open their damn drivers.). And the damn CPU fan spins up far to often!
The biggest showstopper was the lack of Suspend. I really need that when I’m on notebooks. When ACPI (or whatever is needed) works out of the box, a new partition on my iBook is a sure thing for Ubuntu. (And just a matter of disk space, oh well…)
NP: Dan Bern—Too Late To Die Young