Not needed. Regular prepaid cheapo dumb mobiles will work just fine. Google it. There's a long PDF report at http://www.infodev.org/en/Document.43.html detailing one study of use of mobiles for micropayments in the Philippines.

The infrastructure for doing this with bitcoins obviously doesn't exist yet, but I don't see why it couldn't. The *end-user* wouldn't need to have the full wallet functionality on his dumb phone, just as long as the carrier does somehow. All the end-user would need is the ability to send a text message saying pay ___ to ___, and the carrier would do the actual bitcoin transaction and deduct the relevant amount (plus fee) from the user's mobile credit, and credit the payee's mobile credit. Actually, I guess they do that anyway without involving bitcoins at all. I suppose the only need to involve bitcoins would be when the user and payee aren't on the same carrier network.