MustangSVT, SLI Mosaic is currently cost prohibitive because nvidia is using it as an exclusive feature of QuadroPlex (they probably are not happy about eyefinity cutting into their margins there).
But, be that as it may, I very much doubt SLI Mosaic relies on the motherboard to work - it's a feature of the GPU's controlled by software in the drivers.
nvidia could choose to include that feature in geforce drivers at any time in response to eyefinity. My bet is that they won't with current gen cards (even though they are capable), they will have it as a "feature" of Fermi cards (just like geforce 7 & transparency AA -- geforce 6 was capable of it too, but drivers initially didn't allow it on the six series, only the 7 series).
It would take a miracle for Nvidia's GTX 380 to outperform HD5970 with its two GPUs. This kid says no chance in hell, but we'll see.
I think some people overestimate the impact of dual gpu cards on the market. I completely ignore them myself. Compare the single cards, not the duals (or single to dual -- that is just silly).