On Sun, 15 Jan 2012 22:01:05 -0500, Austin Clements <amdragon@MIT.EDU> wrote:
> Quoth David Edmondson on Jan 15 at 11:55 am:
> > On Sat, 14 Jan 2012 10:29:31 +0100, Pieter Praet <pieter@praet.org> wrote:
> > > Might I ask, to what key(chord) have you bound this ?  Due to its
> > > usefulness, I'm inclined to bind it to [SPC], but on second though,
> > > that might be a bit on the intense side...
> > 
> > C-c= globally. That's clobbered in a couple of major modes, but not
> > enough to bother me so far.
> 
> Might it make sense to bind this across the notmuch mode maps by
> default?  This would at least make this feature more visible as well
> as quite useful to people who dedicate an Emacs instance to notmuch.

The elisp manual says:

   * Don't define `C-c LETTER' as a key in Lisp programs.  Sequences
     consisting of `C-c' and a letter (either upper or lower case) are
     reserved for users; they are the *only* sequences reserved for
     users, so do not block them.

...

   * Sequences consisting of `C-c' followed by a control character or a
     digit are reserved for major modes.

   * Sequences consisting of `C-c' followed by `{', `}', `<', `>', `:'
     or `;' are also reserved for major modes.

   * Sequences consisting of `C-c' followed by any other punctuation
     character are allocated for minor modes.  Using them in a major
     mode is not absolutely prohibited, but if you do that, the major
     mode binding may be shadowed from time to time by minor modes.

which I read to mean that C-c= is allocated for use by minor modes. I
could easily be persuaded to change to C-c; and have that bound in our
major modes (and personally bind it globally).