nvidia already has a SLI mode very similar to Eyefinity - it's called SLI Mosiac. Only supports up to 8 monitors in its current form.
Plenty of things we take for granted on the geforce line first appeared in the quadro line (the combined AA modes, SLI AA for example).
Only found this out because nvida asked me to complete a survey and "SLI Mosaic" turned up among the SLI options, so naturally I had to investigate further.
I wonder if nHancer could unlock this for geforce if it was made aware of the feature?