I think this bug may have been fixed upstream a couple of weeks ago when they updated the code to use GtkComboBox instead. See <http://svn.gnome.org/viewvc/evolution/trunk/composer/e-msg-composer-hdrs.c?revision=34620&view=markup> and note this callback:
g_signal_connect_swapped ( widget, "changed", G_CALLBACK (signature_changed), hdrs); -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]