The BlueTomorrow gang go over the basics of A2DP & using the protocol to stream music from your cell phone. It's a protocol that no doubt will become much more prevalent with the convergence of phones & audio players...

"One such feature is Bluetooth’s Advanced Audio Distribution Profile, or A2DP. Basic audio in the Bluetooth specification provides a very narrow frequency response that is optimized for voice communication—great for speech, lousy for music. By contrast, A2DP allows high-fidelity audio to be streamed from one Bluetooth device to another."