The short answer, it would seem after some amount of googling, is that BT headset support for the new OS under development but it is not yet available as there are a number of hurdles to overcome. The page at the Maemo Wiki pointed to in an earlier post contains dead links to code for the old OS.

Some of the problems are outlined here:
http://www.archivesat.com/for_maemo_...ead33483-1.htm

A posting that Nokia is looking at developing support for headsets is here:
http://article.gmane.org/gmane.linux.bluez.devel/6728

When we will see anything is unknown. Please bug Nokia if you want this as I suspect they will prioritize their development based on customer feedback.
http://europe.nokia.com/A4144793
via the Nokia 770 support link here:
http://europe.nokia.com/770