There is not, which is why I asked. Especially since they say the cMP will have support for future GPU's. Bar NVIDIA cards, and they have issues with updates, I don't see it being easy.
Apple's card choices basically decide which AMDs will work. I doubt the 290x will have drivers anytime soon as the thing runs way to hot to be in any of their computers.
Kepler and future AMD cards? That's highly likely, but it'll take a little while.
290X doesn't offer a huge advantage over the 280x (7970) in terms of OpenCL (5-15%). That could be drivers and may be corrected in the future, but we might have to wait for a truly next-gen AMD before we see real gains in this department.
The future could be multi-core GPUs.
You're right that it is somewhat of a leap of faith that we will see GPU upgrades for the cMP, but we have had extremely good luck so far, and this is a lot less of a leap that nMP fans have when they say they expect GPU upgrades in a super-proprietary form-factor.
Anyways, I think this is besides the point. IMO the important thing in terms of this thread is how even out-dated PCIe slots can allow a computer to keep up with a new one. I also think that 2 years from now when nMP users are looking to upgrade to the latest tech, the sting of losing that ability will be more apparent.