Personally I don't think x86 could replace ARM/RISC now or in the future for mobile devices.I won't say can't or never thats just asking to be proved wrong. CISC is just complete overkill imho.

ARM works well, considering its in the iPad and is standard for mobile device manufacturers (people don't know or care for the most part) its dominance isn't going to change for a time and of course there'll be plenty of improvements.

This is being typed from a netbook with a VIA-C7 which isn't perticularly great in my opinion. My sisters netbook with an Atom isn't much better really but thankfully using a form of linux means a lot of these factors can b ignored. it would take way too much effort i think to modify these for a smart phone.

For some extra information what most people call x64 is really supposed to be called x86-64 meaning that its like an extended version of x86, details are a bit tricky. We haven't moved over to pure x64 in the most part (cept for servers and supercomputers i think) as all the old OS and programs would need remade (don't know if completely rewrote or just modified and compiled - this is didn't need to know for my exams so I don't remember exactly) Pure x64 supports massive amounts of ram etc which isn't possible yet.

Wether the the OS used is 8, 16, 32, 64 bit depends purely on the cpu used. I hope I'm not talking down to any one here, and i'm new, but it seems like this is stuff that folk don't know and i think is interesting and useful.