Posting styles is a great thing, but be aware, it requires a recent
version of Gnus.  I'm pretty sure it isn't in the version that's stock
with Emacs and XEmacs.

For what its worth, I found the way posting styles does it's matches
to be somewhat confusing.  After a good deal of mucking about in the
bbdb source, I came up with the attached.  It lets me set my return
address and signature based on the news/mail group I'm in when I start
the message and, if its a reply and the user I'm replying to has
mail-from or signature bbdb entries, use those.
-- 
Dave Goldberg
Post: The Mitre Corporation\MS B305\202 Burlington Rd.\Bedford, MA 01730
Phone: 781-271-3887
Email: dsg@mitre.org