in a follow-up conversation with a colleague, I had a deeper discussion on the evolution of x86 cores and may modify my stance on its applicability to ultramobile devices. The problem I think an x86 faces is expectations, not capabilities. An x86 in a handset would run at 1GHz at best, which is perceived as slow in the PC space. The operating system would have to be optimized for efficiency and power, not just the silicon. The processor would still have to be very stripped down, and highly optimized for micropower management.
So it's possible, but not easy.