[citation][nom]jblack[/nom]Why can't we get this on laptops?[/citation]
You actually already do. AFAIK, Windows 7 tries to only wake up more than one CPU core when absolutely necessary; and, at the same time, Nehalem-based CPUs (and more recent ones) completely shut down unused cores and cache.
If the CPU is basically doing nothing, on these laptops, the only power needed is to support the massive amount of transistors being kept on low-power states (it's a HUGE chip, after all, ARM chips are tiny compared to x86 CPUs).
Now, that being said, it would be rather interesting to see something like a companion Z6xx Atom core (or something similar, since it would probably need to share some logic with the rest of the CPU) on a regular SNB CPU. Those things are like 1W tops, if the rest of the CPU could be mostly shut down the power savings could be sweet. SNB ramps up rather quickly, another core could enable a smoother power ramp.
Cheers.
Miguel