Why do Intel x86 Chips Run so Hot?

Paul A. Houle asks "This isn't a very practical question, but one that a lot of people might have. I've noticed that the x86 architecture and it's clones are real power pigs -- that ARM and MIPS (just to mention a few) take an order of magnitude less power per computation, making them more practical for mobile machines. I'm almost completely ignorant of microprocessor design (I know the difference between RISC and CISC, but not a lot more) and I'm wondering if there is a simple explanation as to why the x86 eats so much power. (And thus, why an intel machine needs to have monster cooling fans...)"