Actually...
the 3870 in the Toms Hardware benchmarks ran pretty close to 8800 Ultra. 3870X2 beats a single 8800 Ultra by a little bit in 1600x1200, and by a small chunk more in higher resolutions with filtering on.
So 2 GPU's is better than one? Big deal.
8800 Ultra SLI and Tri-SLI were not tested against 3870 X2 or 2x 3870 X2 cards in crossfire. So there is no definative winner, except in cost as has been mentioned.
Let's not forget that later this month 9800 GX2 and GTX cards are due in stores...