FUDD
ARM processors and x86 processors are completely different. They use different instruction sets, different pipeline configurations, everything. Intel processors are powerful and fast because they have more advanced instructions for complicated processes, not to mention totally different clock speeds etc.
If you think that you will get the exact same performance per watt from an ARM based computer you are deluding yourself. The trick for Apple will be tuning these ARM based processors for very very specific workloads so that they *seem* to be as fast as an Intel chip when doing certain tasks. That will be fine for an iPod or a phone, but the ramifications for "PCs" is far worse.
If ARM based computers were so desirable, why haven't ARM based servers taken off? Answer: because they are not as competitive as Intel based servers. I am no 'lover' of Intel, but the market speaks for itself.