As Carl stated that he would probably merge this one if someone tested
it, here it comes: I tested it and it works great. When I define a
non-existing directory as maildir it aborts with an error message that
is visible to the user.

I put this in a branch based on cworth/master and with Jesse's
permission, I renamed the functions and did some further cleanup (It's
also integrated in the build system)

The git branch is here:
http://github.com/spaetz/notmuch-all-feature/tree/feature/elisp-fcc

It's not enabled by default. Enable it with:

(setq message-fcc-handler-function 
      '(lambda (destdir) 
         (notmuch-maildir-fcc-write-buffer-to-maildir destdir t)))

(add-hook 'message-send-hook 
          '(lambda ()
             (message-add-header "Fcc: ~/mail/INBOX.Sent")))

in your .emacs for now.

I'll send the patch series by mail in a second.

I have the slight problem that offlineimap takes the message from
INBOX.Sent/cur and stuffs it in INBOX.Sent/new (but that cannot really
be related to this as the mail ends up correctly in /cur after sending
it).

Sebastian