When looking at the benchmarks it's now pretty clear that the huge lead two gpu's have is almost impossible to overcome with one gpu, even next gen. The gtx295 has 2.8 billion transistors compared to 'only' 2.1 billion on the 5870. The drivers got an awful lot better for both sli and crossfire too.
I expect that the 5870 will be the faster card than the gtx295 in all dx11 games however, and I would also expect Catalyst 10 to give the 5870 an edge.
Although the raw speed is somewhat disappointing in light of AMD's own benchmarks, eyefinity is what really matters, at least to me.