I shot the D4s for the first time yesterday while covering a Major League Soccer game and was grinning ear to ear as I edited my images. The files are gorgeous, the AF is amazing, and I haven't even tested the high ISO improvements others are reporting.
I still have my D3s for underwater and it is still a great body but it would be more convenient to standardize at some point, at least until the new body is introduced in a couple years.
I could care less what the pixel peeper testers are reporting; my first experience matched what I was hearing from the sports shooters I know of and respect and the improvements are not subtle.
I don't know if I will seriously consider a housing upgrade but was hoping to get a sense of how dramatic a redesign of the D4 housings will be required in order to accommodate the D4s.