I pulled a new Emacs snapshot from the Git repository this morning and
noticed that typing a space in message-mode fails with a backtrace
similar to the one attached to this message.

Bisecting the changes since my last successful update I found out that
this is the first change that breaks abbrevs in message-mode:

: commit be11c6010ad352ebb93a7921f356a436a6d49f64
: Author: Stefan Monnier <monnier@cs.yale.edu>
: Date:   Thu Nov 19 03:12:51 2009 +0000
:
:     * abbrev.el (abbrev-with-wrapper-hook): (re)move...
:     * simple.el (with-wrapper-hook): ...to here.  Add argument `args'.
:     * minibuffer.el (completion-in-region-functions): New hook.
:     (completion-in-region): New function.
:     * emacs-lisp/lisp.el (lisp-complete-symbol):
:     * pcomplete.el (pcomplete-std-complete): Use it.

The backtrace is attached below, but it includes bytecode bits, so if it
doesn't help a lot, I'll try looking at the changes in a few minutes.