VC firm Andreessen Horowitz just invested $15.5M in ItsOn, a startup hoping to change how carriers such as AT&T (NYSE:T) and Verizon (NYSE:VZ) charge for mobile data. Instead of charging users monthly for a bucket of data good for any app, consumers, carriers, and developers would be able to pay/bill for specific apps over various periods of time. Getting carriers to change is never easy, but the fact AT&T and others have already talked about per-app billing suggests ItsOn's approach could find some takers.