Apple trying to do everything on its own is going to cause its downfall.
This isn't about Apple developing its own CPU from the transistors up. ARM technology means that Apple can license the building blocks of the processor individually from ARM and others to put together exactly the chip they want, rather than waiting for Intel to come up with a generic chip that fits the bill. (Plus, they may already 'own' bits of it, they've partnered with ARM in the past).
ARM isn't getting any slower - the A50 64-bit processors are on their way.
In order of most to least likely:
1. Apple are just hedging their bets against the day when Intel produces the next Pentium 4 debacle.
2. They're looking at entry level laptops possibly running iOS or a 'OS X RT', as a hedge against things like the Asus Transformer, Chromebooks or Windows RT machines hitting the big time.
3. They're thinking of hybrid ARM/x86 machine - think about a MacBook pro with an iPad built into the lid, using the ARM to keep up with email and 'personal organiser' stuff and only firing up the x86 for serious work.