The D40 is not released yet, but will probably be very popular for Nikon as a simplified DSLR for the masses who don't need or understand all the features on even the D50. Nikon developed this low end camera in order to fill this part of the market -- or else surrender it to competition. Personally, I see this camera as a nice compact travel camera on land, but too limited for serious work underwater.
I would bet that a few manufacturers of underwater housings will adapt a housing for it, if there is demand. But it does have a serious limitation: only certain newer Nikon lenses will work with it, specifically the AF-S or AF-I lenses with focus motors in the lenses. Many of these lenses are quite expensive, and have 10 pin connections to the camera instead of the earlier 5 pins. If you already have earlier lenses (like the 105 F2.8D micro), these perfectly good lenses won't work with it. So while the camera body is smaller and cheaper, the cost of new lenses may make the system cost unattractive for exising Nikon users.
So consider the type of lenses you use now and whether they will work on the D40 before getting too excited about this new guy. Unless you are looking for an excuse to go out and buy the newer lenses.... Hey, that's a great idea!
