We've got a lot of discussion around high ISO performance of the D800 vs other cameras - D700, D3, D4, etc. I've got a different perspective and expect the D800 will outperform the D700 and D3 in terms of noise at high ISO levels. In fact, DxO has indicated the sensor scores are likely to be the highest ever measured - but they have yet to perform the hands on review. http://www.dxomark.com/index.php/Publications/DxOMark-Reviews/Nikon-D800-Review
DxO represents the standard for camera and lens testing. Dxo has developed not only a methodology for testing, but also a methodology for comparing different cameras with widely varied numbers of pixels. The approach they use normalizes around a common print size - a 30x20 cm or approximately 12x8 inches - about 8 megapixels. That means images from high megapixel cameras are downsized more than lower megapixel models. In the case of the D800 you have approximately a 4:1 reduction while the D700/D3 have a 1.5:1 reduction.
The other factor is more obvious - it's the characteristics of the photo sites and the relative size of those photo sites. Essentially we have a large pixel pitch approach used by the D3/D700/D3s and D4, and we have a small pixel pitch approach used by the D300/D300s and D7000 - and now the D800.
In the case of the D3x vs. the D700/D3, the low light performance of the individual pixels is much better on the D700/D3 due to the 50% larger pixel pitch. But most of that benefit is lost due to downsizing to the standard image. The D3X ends up with an ISO score of 1992 vs. 2302 for the D700 - or about 1/3 stop advantage to the D700. The D300 score is 679 - approximately 1.5 stops worse than the D3x with similar pixel pitch on both.
But the D7000 fares well. In spite of still smaller pixel pitch than the D300, the D7000 has an ISO score of 1167 - nearly 1 full stop better than the D300.
Putting it all together, that means the next generation sensor improves ISO performance by approximately 1 stop, and the larger megapixels downsized to a standard print pick up 1.5 stops on the D3x and the proportionately larger megapixels of the D800.
Collectively, this means the D800 will be approximately 1/2 stop better than the D700 and D3 in terms of ISO performance - largely because of the large megapixels.
The D4, of course, shares the technology of the D7000 and D800, but has a larger pixel pitch, so you can expect the D4 to outperform the D800 by a half stop. And that will make it a full stop better than the D3 - approximately the amount measured and reported by Bill Claff.