Well some facts.
There are no ARM chip / design performs at the current level that Intel is bring us with Core 2 Duo or SandyBridge. Not even the upcoming Dual / Quad Core Cortex A15.
There are also no Intel Chip / Design that uses as little power as ARM CPU / SoC either.
Apple would properly want a more Powerful ARM or a much less power hungry Intel CPU for Mac.
I think Haswell, the chip coming after Ivy Bridge will satisfied Apple's need. It is a 15W design with GPU + CPU + Northbridge. That is comparing to current 35W design, and 45W design in Core 2 Duo. Eventually I think Apple wanted Intel to move to Sub 10W design, which Intel has states in their roadmap as something after Haswell.
A5 (with integrated GPU and SoC) is 1.9 watts. With process improvement, Apple could probably come out with a 20 core ARM chip (at least 10 times faster then the current chip) that consumes less then 15 watts at full throttle and idles at less then 1 watt.