Xcode would simply spit out a single binary supporting both intel and arm cup's. That part is a meanwhile a no-brainer and requires no effort at all from the developers.Good luck convincing (again) all the s/w houses to write their s/w for a different processor. It would be a suicide. Luckily, I believe this won't happen.
Key would be real fast x86 simulator for legacy software and Virtual Machines. The A8, combined with Apples LLVM JIT technology might actually be able to deliver exactly that...
If they nail that one down, the migration could be seamless for the bulk of Mac users. Only exception I can think of is bootcamp. Anything else, including kernel extensions could be translated on the fly from x86 to arm code using llvm.