From what I understand, the problem in developing an app is that BT is a proprietary thing. With winmo, you can pay a license fee to develop an app, but because of the open nature of android, the code would be out there for all to see and there's no way to license it.
I too was looking for just such an app, as I had it on my Touch Pro and loved it. I used it mainly for listening to navigation instructions, as it was virtually impossible to hear them through my TP's crappy speaker.
I think I found this answer/reason on the android forums site.
Soooo... I went and ordered a stereo BT headset, begrudging, mind you. G
Got a Motorola S9-HD for 39.99 plus shipping and tax. Came to $50. At N1wireless.com Hope I like it.