Interesting idea, but I don't see it happening. The biggest obstacle is the phone itself. The Galaxy SIII has a pressure transducer that measures altitude (including below sea level elevations that only exist in very deep mines), but who's going to produce one with a depth gauge and make it waterproof to 130' or more especially considering that most aren't even waterproof to 5')?
Somebody could write an inexpensive app that would do the computing, but it still needs the depth info. In theory you could build a waterproof housing that has a pressure transducer and connects through the USB port, but I think there are too many problems. First, there are a lot of different phones, so you either make a dozen (or 50) different housings or limit yourself to the market of just a few popular phones. Second, most people upgrade phones fairly often, probably making the housing obsolete within 2 to 4 years. Third, housings with buttons aren't especially cheap. I'm sure you could make one much cheaper than a camera housing, but it's probably still a significant percentage of the cost of a dedicated dive computer.
