Apple is adopting the tech as the supply becomes available, because of the sheer number of units Apple consumes when they switch over it takes awhile to trickle down.
Otherwise Apple might Ship the iMac/MacBook/MacBook Pro all at once -- but that might mean 3 million units in 4-6 weeks from Intel from a product in being ramped up.
So Apple has to do things in stages, since there are almost always delays during ramp up.
If Intel moves quick, we get them quick, otherwise MacBook Pro -> iMacs -> MacBooks get them in stages. Nobody knows what in going on with the Mini, and the MBA with its custom chipset package is on another schedule.