AFAIK the 970 actually has a scaled FSB or a locked multiplier of 2x.
So a Powerbook 970 at 1Ghz would use a FSB of 500Mhz, at 1.8Ghz a FSB of 900Mhz, etc. These are DDR values.
This must make a support chip a little more complicated as it must asynchronously drive the IO bus - but then there is a rumour that Hypertransport will be used and these can have an asynchronous interface.
Re: 64 bit computing
It will probably make little difference in the short run unless you do lots of 64 bit math and don't use AltiVec, as the advantages of 64 bit computing is pretty much lost on a laptop. The thought of having more than 4GB of Ram on a laptop is really cool, but I guess we have to wait for bigger memory modules.
The real advantage comes from the fact that the 970 has a future, or IBM has a future/real interest with PowerPC chips for something more than embedded applications. Improvements to the architecture (hence the "bus speed"x3 response), rather than pure Mhz speed, will make more of a difference.
-Wyrm