If TB is just an extension of a PCIe x4 slot, I have no idea why we won't see PCIe adapters.
It's a good old "screw you" for Mac Pro 2010 buyers.
I believe they've stated that this is because the Thunderbolt bus requires direct access to the motherboard:
(Appleinsider) "Intel says the only way to have it is to buy a system or logic board that incorporates the new Thunderbolt controller chip. That's because the Thunderbolt chip needs direct access to both the system's video and PCI Express architecture."
So this isn't an Apple thing, it's a Thunderbolt thing. Teardowns of the new Macbooks have shown that the new Thunderbolt controller is one of the larger chips on the motherboard.