It seems this issue is related to network-manager, not to evolution itself (see latest comment on above mentioned bugzilla report). There is a patch for this under http://bugzilla.gnome.org/attachment.cgi?id=84286&action=view , which does not 1:1 fit for ubuntu sources. I have adopted it (see attachment) for use with ubuntu revision of libnm_glib.c, after recompiling network- manager the mentioned error-message vanished.
** Attachment added: "Patch fixing dbus_connection_unref warning" http://librarian.launchpad.net/6723940/fix-LP%2389696.patch -- Quitting evolution produces dbus_connection_unref warning in .xsession-errors https://launchpad.net/bugs/89696 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs