The only problem with Nikon is that their fantastic low-light performer (D700/D3) is NOT the same camera as their 24+ MP image quality monster (D3x). I shoot (and love) the D3x, and its image quality is very good up to ISO 1600, but its SPECIAL image quality - the quality you buy the thing for - is at a noiseless ISO 100 (with ridiculously high dynamic range). The D3/D700, on the other hand, have superb IQ up to ISO 6400 (with "emergency modes" still higher) - IF you can accept 12 MP. (Nikon guru Thom Hogan says that he prefers a D3x over a D3 up to somewhere between ISO 800 and 1600, making the same size print, then the low light performance starts to matter more than the resolution) That said, I don't think any of the other "pixel monsters", either Canon or Sony, have any better high-ISO performance than the D3x (the a900 is worse, and I think the Canons are about the same). The only way to get the low-light performance of the low-resolution Nikon FF DSLRs is to accept that their low resolution is part of the package.
-Dan