The Wetpixel home page has an interesting link to an article by Berkley White about the Nikon D3 not fitting in existing D2 series housings:
http://www.backscatter.com/learn/article/article.php?ID=41
Berkley is actually the only person I have so far heard who has expressed a serious interest in using this camera underwater (as opposed to one friend I have who want to house the D3 because he must have the latest and greatest toy!). For most Nikon users the D300 seems far more attractive (small size, better AF and improved view finder and better high ISO noise control than all previous Nikon DSLRs to date) for underwater shooting.
While all serious photo (underwater) D2x owners who I have seen or had emails from since the D3 was announced have said that they would rather wait for the D3x (the 1Ds3 rival) version as high resolution is more beneficial to them than high ISO - which has only limited uses underwater. Most of the time we shoot at base ISO. Also FX format won't allow them to use their Tokina's 10-17mm which everyone has fallen in love with of late. Rumours are that Nikon will announce this camera (about 20-24MP) in the first half of 2008.
But maybe it is not that simple. First I think that Nikon users need to be more open to the capabilities and possibilities of high ISO. Put simply we have never had it before, and those who have tried the D3 (there have been some interested tests posted by Canon sports shooters) say its high ISO performance is a stop better than Canon's 1D3. This is in a totally different league to anything Nikon users have had available before. Maybe we need educating and the chance to think about the creative possibilities of high iso underwater.
That said, while I have seen many Canon 5D users regularly shooting at ISO 800 underwater, I am yet to see great competition winning images shot at high ISO - that are redefining what's possible underwater photography. But with the Wildlife Photographer and Antibes coming up in the next month maybe I will then?
Another point to consider is that there are a few rumours that the D3x version of this camera may be modular and therefore won't automatically fit in a D3 housing. Meaning housing the vaporware D3x could easily be 12-18 months or more away. Plus if it is modular it might have less good weather sealing and could be $10,000 USD.
Lots to discuss.
Alex