A couple of recent examples are Thunderbolt and the unreleased Z68 chipset that is in the newest iMacs.
I really doubt this will happen anywhere in the near future simply because ARM chips don't have good performance compared to x86 chips. Most people would be unwilling to accept a performance downgrade.
I could see the MacBook Air line and other consumer products going in that direction, but certainly not the MacBook Pros or the Mac Pro desktop for that matter. These products are targeted towards serious professionals who need performance. The notion of ARM chips in a workstation computer is laughable for the near future.
There's pretty much zero chance ARM will ever reach performance parity with x86. Not to mention Intel's near two year process technology lead over everyone else.
The only scenario I see something like this happening is far in the future, when everything transitions towards the cloud and thin clients. But for now, we need real computers with real performance.