My understanding is that Apple's A-series SoC for iPad/iPhone are great in quick bursts, but not so great for holding that performance for long-running processes. The peak performance is way up there and well enough for a desktop/laptop, but Apple will need to make some modifications to make it run as well for longer periods of time without the big dips.
Like someone said above... the rumor is that Apple is working on ARM-based CPUs for future Macs. So, we're sure to see what they've done. If true, it's very likely a variant of their A-series SoC. I could definitely see this used in Macbook Air. It'll be interesting to see the GPU options in something like a MacBook Pro, Mac mini or iMac.