There is still no technical reason for SLI to not actually work. So it's my understanding that nVidia are just doing a better job at locking it down to nVidia only motherboards with more artificial locks built into the drivers.
I've read that they are using a special encrypted key on their nForce chipsets which tell the drivers to allow SLi to work, it's only logical that they've incorporated something similar into the new graphics chipsets to stop something that should work from working.
Just because they haven't done it yet doesn't mean it is impossible, just too hard