All that would require is Apple to say that all ARM iOS apps don't work on x86 version of iOS. It's not like Apple currently even guarantees an App from the store will work between iOS versions. The need to update apps for an architecture change would simply be pushed to the developers, just like it is now for iOS versions.
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.
I work for a device manufacturer that went with Intel for a product - and x86 is a huge problem regardless of Dalvik.
The fact is, most popular apps out there, and essentially all games, have a portion of their code running in JNI (C libraries called by Java) for performance reasons. It pretty much wiped out half of the Android Market as incompatible and had software partners screaming at us.
However, it's unlikely Apple would switch to x86 unless Intel was willing to license the spec (Intel would much rather make the chips); as ARM licensing allows them near-full control over their chipset vs being dependent on a third party.
Intel's lack of customization is the real problem. NVidia, Apple, Samsung, TI, and Qualcomm can build amazingly specific chips to the task of mobile, while Intel wants a one size fits all solution. Intel has effectively removed all the chipset makers from the PC world giving us poorer performing PCs. This approach won't work in mobile.
Beyond that, you have a number of vendors able to use ARM cores to make product. Never mind AMD getting using ARM cores. Intel has a chipset they want to sell everyone. The ARM ecosystem consists of multiple solutions from multiple vendors all with the core CPU software being the same.
Not to mention that now Apple has almost full control over their chip, so why would they give that up? I don't know what Tim Cook will do in the future, but if Steve Jobs was still alive, I'm pretty sure he would've switched to ARM for Macbook Air or perhaps a "next-gen Macbook Air" to a 64 bit ARM chip in 2014-2015.
Look at the new iPad and its ARM chip. It can already support well enough a 2k resolution. Can Intel Atom do that right now? I doubt it. Medfield, their "low-power" Atom that is supposed to compete with ARM, can only support 1024x600 right now.
If Intel had taken low power seriously back in the days of the Newton and the StrongARM (which Intel actually owned for a while) they might have won the Apple business instead of ARM back in 2007 but now it is probably too late.
Apple has switched CPU architectures before but never with the number of applications that they are currently dealing with in the App Store.