I'm making it a webapp. If you look at my profile you'll see I do tons of mobile work but I think something like this is better suited to a computer than a phone.
For mobile integration I highly recommend a CalDAV[1] feed. This sort of budgeting really makes a calendar view essential, and CalDAV integration with the phone's own calendar seems ideal.
Beyond that, some minimal mobile access would be nice for things like adding/modifying recurring events, updating the balance, or entering a new one-off debit/credit.
I stress this because I think for a lot of people budget applications provide a peace of mind, and it's nice to be able to check in on and confirm these things while you're on the go.
