Apple's phenomenal success with iPhone later iPad and complete focus on iOS and putting OSX into back-burner, acquiring two of ARM startups/companies are some of the symptoms Apple is showing that it is planning planting ARM based processor into OSX hardware.
The real highlight is Apple TV, it is moved away from Intel to Arm based A4 chip. Recent release of ARM A15 (Eagle) Multicore chip is real highlight. Couple of working NVidia Tegra mulit-core chip (though it is not A15 based) tablet tells that ARM 15 is right choice for Apple's Mac portfolio. The good thing is ARM A15 is multi core which can be scaled from single to many. Apple was not afraid of moving from IBM PowerPC to Intel, same thing would happen once again from Intel to ARM A15 chip. My guess is 2012 Mac line would be refreshed with ARM based Apple's own chip.
In between, Apple may try AMD with its fusion chip.