If you are still using Mac OS Tiger (10.4) and haven't upgraded to Leopard (10.5), there are two workaround solutions to get A2DP (stereo bluetooth).

1) Softick Audio Gateway for Mac OS X
This is the easiest to use solution to get your a2dp fix on your Mac. You can download the beta version for free (unlimited duration, but only works on Intel Mac). It installed easily and
has no problem detecting my stereo bluetooth headset (Sony DB-BT22).

A final version is also available for purchase at 19.95 USD. However, it seems that the development for this software has died along with the support of A2DP in Leopard. There is scarce information about the difference between the beta and final version. I would assume that you get more control with the purchased version.

Compared with solution (2), the beta version do skip when playing music on my iTunes. You'll get about a second of no sound every once in a while. The final version might have solved this problem, but I believe the skip is mainly due to the bluetooth bandwidth problem (older bluetooth specification, especially version 1.0+).

Lastly, solution (2) seems to have a better sound quality than the softick audio gateway.

Solution (2) will follow shortly after this piece. Stay tuned.

Download Softick Audio Gateway for Mac OS 10.4 (Tiger)

Click here to read about Solution 2