I hacked together the attached elisp yesterday.  It provides bindings to
put sender or recipients into bbdb.  (Recipients part needs more
testing.)  It also colors the from line green if the sender is in bbdb,
or orange otherwise.  When it's been through a bit more testing, I'll
submit at least the first part as a patch.

I prefer not to autocapture everything into bbdb, and for the same
reason, I don't want to use notmuch itself as my contacts DB.  Mostly,
this is because I read lots of email on lists, sent by people I'm
unlikely to write back to.  I don't like every John I've heard from
coming up on autocomplete.  Other reasons include sync to phone and
adding contact information from channels other than email.

bergey