I have recently stumbled upon enabling Stereo Bluetooth for music in a headset. Here are the steps. It's not very great for controlling tracks but it works.

1. It only works with the sim card removed.

2. Make sure U connect your blackberry to your bluetooth headset.

3. Open up your music folder and select "play all".
-I select "shuffle" to have the music play random.

4. Press the "home" button(red/end call) to get to the main screen once the music is playing.

5. Ensure the music is still playing then press the "call" button(green/call) to bring up the call log.

6. Once U have the call log screen open, press the call button twice, very fast. It will give you a message "Emergency Calls Only" and give you the call log on full screen.

7. Then U will hear the music playing on your bluetooth headset.

8. U can only control the volume, and if you press the "home"(end call) button it will stop streaming to the headset.

Since this is possible, I would imagine that someone could come up with an app that does this without having to remove your sim card. Someone with 3rd party app program experience should be able to help us out to comeup with a better way to stream music through a bluetooth headset.
P.S. I use a Samsung WEP200 Bluetooth headset.

I only have the pearl so I don't know if this works on other Blackberry models.

Is anyone having any luck getting this to work for them?
How about a programmer coming up with an app that can stream and control the tracks with a bluetooth headset?
Hint:It gives you a "New Call" full screen while it streams.

Check this thread for instructions on how to do this without removing the sim card. The instructions are the 2nd to last post in the thread:

Stereo Bluetooth Headsets

I moved to an area where T-Mobile doesn't get service, so I am roaming 24/7and I am not able to use the browser. So again, this is a method to use if you really want stereo bluetooth with your pearl and do not have a data plan.

Has any "programmers" been able to come up with an app yet that can accually make the pearl "Bluetooth Stereo"?

Not yet. Rumor has it that it may be included in the next major release of the OS for the Pearl.