A friend of mine just told me about USSD. Of course, you already knew about USSD – it’s been in the GSM spec since the beginning (like SMS). It’s a sort of session-based SMS. It has a few current uses – like querying your pre-paid balance – but no economic model (there’s no way of billing or metering USSD traffic at the moment and no handset-to-handset functionality). You could use it, though, to trigger data delivery back to an app running on the handset, for instance. Or to request location data for a navigation app. Very interesting. Expect a rash of USSD apps over the next year or so as mobile entrepreneurs push the boundaries of voice, data and SMS. Imagine: an unexplored platform for mobile business!