There are housings and hardware to turn your smartphone into a dive computer.
The iGills works with the iPhone 3 and 4. It's $329 (plus, your iPhone).
The DivePhone is an Android/iPhone app and hardware. It's more expensive at somewhere up to $500.
I just wish the Garmin 401 were more waterproof than 1 meter.
I'm thinking maybe with a drybag?
Of course what I really need is this full-face mask with regulator, LED lights, and radio.