I do have to wonder now how long there will be Mac versions of PCI/PCIe video cards now with the nMP change. You might buy a cMP now, and not be able to get a new video card in 2 years...
I have only an amateurish understanding of this, but I believe there will almost certainly still be cMP PC card options for the next two years, for the following reasons:
1) AMD and Nvidia drivers are typically universal drivers for entire families of GPUs. So including drivers for any AMD/Nvidia GPU that comes in a Mac brings compatibility for a slew of other cards.
2) I think self-init is unlikely to go away in the next two years, which enables PC cards to use these drivers, but even if it does go away, there is still ATY_INIT which does the same thing.
3) Firmware ROM files are derived by some hard working folks from similar-model AMD and Nvidia GPUs in Macs. Unless Apple goes all-Intel in the next two years, I don't see this changing. Even if it does go away, many people don't even bother to flash in the first place and live without boot screens.
Honestly the only problem I can think of in the next 2 years is that GPUs are ever more power hungry. What's next, 10-pin power connectors?