See meeting minutes at: http://www.w3.org/2007/07/25-xmlprotocol-minutes.html#action06

WG resolved to open and close with no action. The WG felt that while Mark's point was possibly technically correct absent the context of the MTOM Policy assertion that using the application/xop+xml media type did not in fact represent an endpoint's ability to support MTOM, but rather the XOP media type, which is not what the MTOM policy assertion is about. The WG also noted that in fact the multipart/related; type=application/xop+xml did not technically represent support for MTOM, but could, in fact, represent another serialization of a multipart message that had, as its root body, an application/xop+xml serialized entity. However, in the context of use of the MTOM policy assertion, one could reasonably infer that it meant support for MTOM if the receiving endpoint of the ACCEPT header had exposed a policy that included the MTOM policy assertion.