There is a t least 5 manufacturer involved in making a housing for the 3D, the newer D3s has some slight physical differences, this prevent the standard D3 housing from being used with the D3s, at least as far as video is concerned. as a result our standard D3 housing is no longer produced and has been replaced with a newer D3s version that will accept all 3 versions (D3, D3x & D3s), expect other manufacturer to react in similar fashion and price point to remain similar.
I still lean toward the Nikon D700 for underwater FX format, the body/housing package is way smaller and easier to handle, but that would mean getting another extra camera body.