Those X shaped things are on the back of almost every AMD GPU I can think of.
Even lowly 2600XT from 2008 pro has the.
Image is from 7970s.
Historically, it has been rare for AMD/Nvidia to get one of their board partners to bother with Mac cards. In the past only mild changes had to be made to existing PC designs, but even these mild changes required retooling and reconfiguring PCB lines, so was rarely done.
The Quadro 4800 Mac version lost 2 DisplayPort connections and just shipped with Dual DVI.
The Quadro 4000 Mac they just loped off one DP connection.
The EVGA GTX285 Mac had a different EEPROM and no S-Video port.
The AMD 3870 was originally going to be a completely different card but somehow that got cancelled and we ended up with the CHEAPEST version made, identical to PC version but with different EEPROM, no S-Video, and 1 little resistor moved to stop flashing.
With 4870/5770, and 5870 AMD cards they added MDP ports and lost a DVI or two.
With these new cards, they appear to be a completely different design than anything shipping anywhere on the planet. Instead of changing a port or 2, the card is a completely custom PCB.
If Apple doesn't hand out the plans for free, someone else would have to completely re-engineer them. And if Apple hands out the plans, you would still need a special PCB from ground up. ie, you won't just shut down your FirePro line, teach the robots to switch a few pieces out and fire the line back up. All to bring you the wonder and delight of Thunderbolt.
I don't see a 3rd party even considering this process until they see if these new Mac Pros are selling. This will keep the prices of these cards right up in the stratosphere, where Apple likes them. (Their 5870 is still $450 while a PC one is worth about $150 now)
There is no way in high holy heck that Apple will let you switch a component that requires re applying thermal compound, so easy to do wrong and fry stuff. So, what GPU you buy your Mac Pro with is likely what will be in it when it heads to the landfill years down the road.