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