I've always found it surprising that Apple did not have support for the A2DP Bluetooth audio profile in Tiger. A2DP is the profile for stereo music, as opposed to the lower quality mono profile for standard phone headsets. It was rumored, but not confirmed by Apple, that Leopard would fill this hole, and now it appears that's exactly what has happened. Ryan at Thoughts Abound has successfully set up Leopard with his Motorola S805 Bluetooth stereo headset to stream music wirelessly from iTunes. Not only that, he notes that "AVRCP is included as well so you can control your music from the controls on your headphones or stereo."
Unfortunately, A2DP compresses the audio, so it's not exactly a high-fidelity solution. But if you really want to listen to your tunes from your Mac without wires, this is probably the way to go.