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.