I don't think it would take much to recompile existing apps but there are so many apps, just approving them would take Apple years.

Then think of the all the apps that are tuned to take maximum advantage of the platform. That code may work but it doesn't mean it will work optimally.

Intel is going to have to do a lot better than "we win some, lose some" on battery and performance to convince Apple to switch.

The last time Apple did this apps didn't have to re-compile - the Rosetta translation layer did a pretty decent job. If Apple were to switch architectures (rather unlikely, but still) I imagine they'd be providing a similar sort of translation layer for most apps to use.

You aren't going to use Rosetta style software on a battery constrained mobile device. There would have to be a huge benefit to switching to make that kind of problem for users. The iPhone is already bashed for having battery issues. What would be the benefit of switching to Intel for Apple?

They could even ship a device with an ARM core in addition to the Intel core, and only activate the Intel core for games etc.