People have tried overclocking with varying degree's of success(no one has actually /confirmed/ it as working yet though). Also, overclocking while undervolting sounds like its not possible, though I may just be ignorant of it, but the usual method of overclocking is to increase the voltage to the FSB.
There is under-clocking, but eventually that will get to a point where you will get no gains, the processor is already under-clocked to 800Mhz, much further and you probably wont gain much if any battery life back.
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