It's not so much a matter of capability as it is a matter of carrier restrictions. The carrier restrictions keep an app that does this for the T-Mobile G1 from

It's not so much a matter of capability as it is a matter of carrier restrictions. The carrier restrictions keep an app that does this for the T-Mobile G1 from being easily distributed (and I think it requires rooting your phone, last I checked). So the amazing gadget already exists, it's the contractual agreements that need to be made amazing.