The official tool is available here:
http://trifinite.org/trifinite_stuff_carwhisperer.html
It has also been ported to the Nokia N810:
http://talk.maemo.org/showthread.php?t=20780
I’ve downloaded it and installed it on my Nokia N810, but haven’t had a chance to test it since I don’t own any bluetooth headsets.
More to come as usual..
#Update 11/17/2015
It appears that video no longer works, this should suffice:
Eavesdropping on Bluetooth Headsets by DelwinWood
#Thanks Patryk