I vaguely recall reading about someone developing a chip that could run both ARM and x86 code. The idea being that it would run ARM code most of the time at low power, and then power up in order to run x86 code when necessary. I don't know if there was any clever sharing of the silicon, or if it was in fact just an x86 core and an ARM core glued together.

The only thing I can find on it though is this IEEE document reference: http://ieeexplore.ieee.org/document/6136696/