I'd find this useful too.....I have an older car, older stereo etc....
I love the app that routes phone audio to the FM transmitter, works great but I no longer use my N900 for phone. I think if someone would write a client to accept A2DP data then if the FM transmitter were enabled it would just work. I've been playing with this the past day and just by typing "sdptool add A2SNK" Advanced Audio appears as a service to bluetooth peers. It connects but after receiving a few packets it disconnects because a lack of client. If someone could code the client it would eliminate a need to purchase a hardware audio gateway and it would make the N900 cooler in general.