No real suprise Dave. I have to say the D3 series is amazing when it comes to noise, but physics is physics.
After using the MK4 for a bit, personally I would not shoot over 1600 for birds but I have not been shooting in the best light and I may change my mind in the future. It is so lovely at 200,400 and really nice at 800.
For sports and people I would have no issue cranking it and applying NR, so for 95% of their market it is probably perfect.
In a similar way Lou, I feel the same about the D3/D300. D300 for me being fine up to 800. pushing ok at 1000: beyond and the D3, but as you say, for 95% of the time will do nicely for most apps. Haven't felt the need for the D3s although looking good as a better purchase for me will be the D3x, providing differing options. for once, it looks like we are being spoiled for choice. :)
I have the Mark IV since the last few days. I have been intentionally testing the high ISOs. I am ok with ISO 1600. I have got a few ok shots at ISO 3200 as well. However, I need to do some more tests.