UPDATE:
http://www.semiaccurate.com/2010/02/20/semiaccurate-gets-some-gtx480-scores/
The GTX480 is 5% faster than a 5870 on average. So worse than we thought.
The GTX470 is 10-15% slower than that, so probably has a slight lead on the 5850.
Given the massive die size, yield problems and power issues, I don't believe there's a price Nvidia can put them at that is both profitable and competitive.
Also another issue: the chip's DX11 support is not ready. It has image quality issues on parts of the DX11 Unigine benchmark.