Companies like Asus/Silverstone (and more who don't spring to mind right now) have created Thunderbolt GPU enclosures to allow laptops to do 3D work. However Intel won't let them go to market because they don't have OS X drivers, and Apple has no interest in supporting Thunderbolt GPUs when they cold be selling people additional Macs instead.
The GPU enclosures have been held up by Intel refusing to license them but not because of osx drivers, there are licensed thunderbolt peripherals that do not have osx drivers.
But there hasn't been an official statement about why they've been refused from Intel, so I can't link to it. You mention you still believe Intel has refused to license the hardware - what reason do you think applies?
Having ExpressCard in the middle kills your bandwidth, and ExpressCard is way narrower than PCIE or Thunderbolt.
One of the reasons I don't buy Macs, even though I like Mac OS X, is the fact that only high end Macs have proper GPUs for serious 3D programming.
The developers I work with all love their Macs, but they hop onto Windows for serious Unity work, just so they can use Visual Studio with ReSharper instead of MonoDevelop.
The gaming community has a different mindset than UNIX or FOSS communities and Mac wasn't never good at games (at least in Europe).
The rise of consoles happened in parallel with the downfall of Atari ST and Amiga.
So eventually the devkits for consoles happened to exist only on PCs.
Also the demoscene groups started to focus on the PC as well.
It doesn't help that Apple, except for the Pippin and now iOS, never had a gaming culture.
So you will never see a Mac game ring with subzero temperature cooling system and a pair of SLI graphic cards powered by their own power plant. :)
I'd counter by arguing that the computer brand that still to this day laymen consider "better for graphics" would find value in supporting a spec that's very unveiling discussed the possibility of external GPUs.