According to Apple's iPhone 3G product page, the mobile version of the iTunes music store will remain accessible only over the phone's Wi-Fi connection, and will not take advantage of the new 3G capability. I guess it shouldn't come as a shock that the wireless version of iTunes (dubbed "iTunes Wi-Fi") should remain a strictly Wi-Fi service. Still, it does seem like Apple has missed an obvious opportunity to allow users more ways to purchase music.
With services such as Napster Mobile and Rhapsody already allowing wireless music downloads over 3G networks for many iPhone competitors, Apple's reluctance to jump on board likely comes down to an unwillingness to share per-track revenues with AT&T. After all, when you own the music service and the hardware, why concede any profit to a wireless carrier if you can help it?
Only Apple knows for sure why music downloads have been locked out of the iPhone's 3G capability, but personally I'm disappointed to not see it included. The iPhone is so close to becoming a great music discovery tool, but limitations such as this and lack of Flash audio support are big barriers for online music fans. Oh, and don't get me started about the lack of streaming Bluetooth audio.