Media Button Router works with your Bluetooth stereo. Gone are the days when you were limited to a single app that would launch and play music when you hit the play button on your stereo. This app lets you, the user, decide what app should play music when you hit the play, previous, or next button on your stereo. It prompts you with text-to-speech, and lets you select by hitting previous to go back an app, next to go forward an app, and hitting play to select the app that you want to use. If music is already playing, Media Button Router stays out of the way, and lets whatever app that is playing the music handle media button presses. You can now have a fully hands-free music experience.
Please note that this app is targeted towards Bluetooth devices that have at least Play/Pause, Previous, and Next buttons. Also note that this app will not be compatible with all music apps. It has been tested, and works with at least the following apps: MixZing, PowerAMP, ³(Cubed), Android’s default music app, Pandora, and the latest version of Amazon MP3. It works most of the time with Google Listen and Rhapsody. Winamp and Doubletwist are not supported. In order for your music apps to work, you will likely have to enable a “headset controls” option in each app’s preferences.
Please report any issues at http://code.google.com/p/media-button-router/issues/list
Also, please free to submit a patch or contribute to this project, it's all open-source. I no longer use this app daily (my commute has changed), so any progress is going to come from YOU.