I'd expect there to be pretty close to parity on the CPU and support chipsets from the CPU manufacturer. It becomes less of an issue as more stuff gets integrated into those chipsets. If you want to play with the bleeding edge stuff, you're probably out of luck.
As long as they stay focused and committed to the platform, they won't be too far behind. If they decide there's more money to be made in pocket toys you could find yourself falling behind pretty easily. When Jobs retires, the next guy might have different ideas.
That's the thing, Apple seems to get the bleeding-edge CPUs and chipsets before anyone else, retail or OEM. I tried to do a price comparison when they last upgraded the Mac Pro, but I couldn't find the chip at that FSB speed anywhere else. They showed up at Dell quite a while later in their workstations (which were more expensive than the Mac Pro BTW). Apple also got the small form factor CPU for the MacBook Air long before it was available to anyone else, although IIRC Apple asked Intel to design it so you'd think they'd get it first.