If Windows 8 RT can run on an Arm v7 + nVidia GPU ( Tegra 3 ) then OS-X can run on an ARM v7 + PowerVR GPU ( A6 / A6x ). The ARM processors are very efficient and can be scaled up to use more power available in desktop or laptop configuration. Processing-wise, the 4-core @ 1.6 GHz plus one light-duty core configuration in Tegra 3 can process a lot of data. It is only the GPU that will hold these systems back compared to desktop systems. Even the Tegra 2 is capable of pushing HD video to an HDTV through HDMI. Apple can still use an ARM processor as a platform and have the ability to add a video card like traditional systems.