Alex
I've summarised a few points I'd make below:
1. I'm not sure that many Pros will house a D3 unless they actually plan on using one topsides - in my experience most Pros use a 1DS series camera underwater because this is what they already own/use above water! I have said it before and no doubt will again, that I PERSONALLY know no Pros using Nikons at the moment out of choice over the 1DS series cameras, and do know many who have shifted from Nikon to Canon. The reasons are varied and I'm not going to go into them here, but Canon do seem to have the high ground at the moment and Nikon are playing catch up - and please accept that I'm not trying to be controversial merely factual!
2. The underwater photo world is conservative and traditional (yes I'm now being controversial!) in that Nikon have been dominant for a long time because of ttl Nikonos fitting flash units which originated with the Nikonos 5 and its horrid flash connector and then became the norm with film SLR cameras resulting in Nikon having number one position underwater. This is now changing slowly in as much as Canon dSLRs are starting to show up more in housings and the compact market is shifting the traditional allegances with regard to those who upgrade to dSLRs.
3. The D3 has a lot going for it in terms of lenses for potential wide-angle use underwater. The 20mm and 18mm lenses offer the potential for careful optimisation behind dome ports which could seriously challenge Canon's existing lenses for high end use, however.....
4. I can see lens flaws using a 1DS camera both above and below water and adding in a dome port simply makes things worse. Increasing MPixels gives better central image quality but does not deal with edge quality issues so I'd actually say that the D3 currently sounds like a well and viably specified camera to house, but....
5. Nikon do need to deal with their macro lens line up - I hear many complaints about the new 105 VR not being as good as the older lens (although I haven't tried it myself) and the used price of the none VR has increased (if you can get one). The 60 is a superb lens but is looking old and needs to be replaced with a faster internal focus version. And...
6. Just how many shooters will see advantages of the D3 over the DX series will depend on how and what they shoot and if they do see FF advantages for their style, they are probably already shooting Canon!
Lastly, I personally would be interested in a housed D3 but am far from sure that it would provide sufficient useful advantages to really make one worth my while buying. And a rumoured D3X has severe competition to face in the 1DSMkIII whatever its high ISO advantages as Nikon will have to provide a full series of high quality 'L' type lenses to compliment it and compete with those available from Canon.
Just a few thoughts!!!