Sadly the networks still work in a semi walled garden mentality. It is not just the data plans but also access to the 'infrastructure' that inhibits a certain amount of 3rd party service development.
we have had no end of ideas, and a certain level of success in prototyping some of them, but to put them into production becomes prohibitive, especially if you have to go through making deals with the network operators (ie the cost is not in development but in the business and legal model).
All the networks (in the UK) do offer developer forums and i think some are moving over slowly, but they see this as releasing a major asset. Remeber SMS made them money. They bought the kit and then kept the profits. Now we are talking about sharing those profits significantly with 3rd parties and at the moment there is a reluctance to do that.