I have no problem with SLI per se, but I'm just not seeing it as an attractive option in terms of price/performance, even if it works with 6800NUs. 1.75 times a 6800NU's performance (assuming that NVIDIA's performance estimates are realistic and pan out across the board) is not going to be worth paying a premium over a 6800U/X800XT AND having to buy what is likely to be a very expensive dual PCI-Ex16 motherboard. And while you may be able to get some scary FPS numbers out of two 6800GT or 6800U cards, the price for such a setup is far too high for all but a handful of PC enthusiasts.
I think everyone has gotten spoiled with being able to build sub-$1000 systems. Honestly, there are people are are willing to invest $2000+ into a high-end system. An additional $400-$500 for a second video card is not that far out of line. An AMD 64/3500+ with 1GB of mem, 250GB HDD, DVD-RW, nice mobo, and a pair of 6800s is still cheaper than a Mac.
