Custom Jalapeno chip with a whole suite of Wallet apps, maybe even some Satoshi Dice software as way to get some investment from them. Id buy one that's for sure! You could use a wireless connection to mine and your phone is normally always on so that would mean your mining while in range of the wireless connection. Also the Litephone a cheaper version could be released in conjunction with the Bitphone, bringing Cyrpto Currencies and mobile user based banking to the World.
The battery would run out, mobile devices aren't really for mining.