Nothing to do with cell phones, but.. Anonymous P2P sharing
The world needs that like you wouldn't believe.
That's what I plan on doing, but hey, if someone can get it done before me, it'll benefit us all!
As for cell phones... there's a lot you can do with that. It's hard to come up with something at the drop of a hat, I know, but... try to merge the portability of the technology and something that you do in everyday life that could be made easier by integrating it.
For example, I made a webapp that would give driving directions in the event you became lost. All you had to do was know your current location (address, city) and your destination, and it would spit out turn-by-turn directions.