Since Intel “owns” Moore’s Law, and are transforming Atom to a first class citizen building on their newest processes all while Ivy Bridge and later Haswell are approaching extremely low power consumption, ARM will soon start to feel the heat.
So I would say, that x86 tablets will have great battery life in the not to distant future. (Hell, Atom Tablets with as big a battery as the new iPad, already have equal to better runtimes, because the new iPad has high power demands)