The version of the IDE driver in 1.5.x has some timing problems with
the CD on the new iBook. It's fixed in revision 1.16 of
src/sys/arch/macppc/dev/wdc_obio.c.

Installing over a network is the straightforward workaround, although
the resulting 1.5.x system will still have problems with the
CD. Unfortunately, there isn't a snapshot recent enough to have the
fix in place, but you could install 1.5 over the network and then
move to -current.

        - Nathan