As this pertains to Apple:
A) It's really only 5GHz. The 6GHz is "in the lab" which translates to no chance in hell of seeing it in a shipping product. I'm sure IBM had 3GHz PowerPC 970s in the lab too.
B) Due in mid 2007. Great. What was Apple supposed to do for a year and half?
C) It's a big, hot server chip. Not that the G5 isn't also a big, hot server chip requiring liquid cooling for higher clock speeds.

But in a Powerbook? Nope.
D) Apple never even got chips based on the Power5. The G5 is based on the Power4. Power6 is looking pretty irrelevant.
E) IBM made it pretty clear that they were interested in developing server chips for their own machines and chips for game consoles. But desktop and laptop chips for Apple? Not so much.