Found and tested this freeware utility :

With this utility for Windows Mobile 5 devices you can play sound from your smartphone through your bluetooth carkit.
I've tested it with my HTC TyTNII (WM 6.1) and a Parrot CK3100 bt carkit and it worked. Of course it's not stereo but it does the job.
With the prices of memorycards droppping I thought it might be interesting to mention on the forum.