Cingular, the only 3G provider without a music service, is planning to release a music service (we don't count T-Mobile because their service is only hypothetical until we can use it). Cingular plans to officially announce the service tomorrow, and will let users transfer songs from Napster, Yahoo, and various subscription services to users' phones. In 2007, Cingular will allow Over The Air (OTA) downloading directly on phones, so you can rock out on the go.
So what's different about this service than just uploading songs to your phone, something you can do already? Cingular plans to also release software to make it easier for people to transfer songs, and to "download them wirelessly". Not quite sure what this means, but it's probably Bluetooth file transfer.
Is this service something in high demand? How often are users using mobile music downloads? Maybe if it's synced up to your iTunes account and you simultaneously grab songs on the phone and your computer, that would be useful.