Posted on XDA, but thought I'd share here too. If I don't answer you here, try the XDA thread 
My first Android application!
BTmono
Description:
Simple app to stream audio to your bluetooth mono headset.
Released as donationware, so if you like it
show your appreciation
Requires Android 2.2 or higher. Tested on stock HTC Evo 4G ROM (2.2) and CM7 nightlies.
Notes:
- Turn on bluetooth and set media volume to desired level before running the app (will do this automatically in future releases if there is demand for it).
- This version contains a workaround for a bug in Android 2.2 which causes a system crash under certain circumstances. The bug has been fixed as of Android 2.2.1, so future releases will include an Android version check and alternate (better) shutdown method for 2.2.1+.
To-do:
- Check if Bluetooth is enabled at app launch (and enable it if it's not).
- Widget. Thanks pure_energy1 @ XDA for reminding me.
- possible (if requested) - disable bluetooth when you turn off streaming.
- possible (if requested) - set media volume to predefined level before enabling bluetooth stream.
- Version check and different shutdown for 2.2.1+
- Other features?
