All that all of you talking about implementation MMS support is dirty hack.
1. MMS is not unique service that requires separate interface. Just see the OMA and 3GPP's R5 requirements. All operator's services should be provided thru IMS. MMS is one of. So, the MMS feature is SIP.
2. IP-address spaces of Internet's connection (Inernet's PDP or Wi-Fi) and operator's PDP could be the same. There is collision.