[link|http://www.theinquirer.net/01020211.htm|Apple could easily migrate to X86].
Much of the code is portable, and Apple has experience running on different CPUs. But it doesn't make sense to run MacOS on a new architecture unless the existing CPU architecture has run out of steam. This is because some of the MacOS would have to be emulated until all the code was ported. Without a substantial speed delta in going to a new CPU, there won't be any performance benefit.
So don't expect MacOS-X/86 any time soon.
Cheers,
Scott.