POWER5 and POWER6 have evolved quite a bit from where
POWER4 was.  IBM
 made a lot more money selling POWER processors, systems, and consulting
 to go with it, than they ever did with selling PPC chips to apple.  IBM
 was dragging their feet with PPC features on the G5 (and whatever would
 have been G6), which is why Apple abandoned them; if IBM was making
 enough money from Apple on their G5s,  I'm sure that they would have
 payed more attention to what Apple wanted (like low-power noteboot
 CPUs). 
Looking back, the POWER lines for Apple were starting to be more of a
nuisance for IBM. I do not think anyone was crying in Fishkill when
the news came down about the Mac architecture change.
--
Will