T-Mobile now allows MMS messaging to Google Voice users
As yet, the only way for a Google Voice user to receive an MMS was if it were sent by a Sprint user. Supposedly, if a Sprint user were to send an MMS to a Google Voice user, it would be routed to Gmail. Of course, in practice it hasn't always work that well. Today, a Google employee, Alex Wiesen, has said that the same support is coming to messages originating from T-Mobile users. The Google+ post said:
Of course, all of this is talking about MMS being received by Google Voice users and not on a way for GV users to send an MMS, which is currently impossible. Perhaps Google is waiting until Verizon and AT&T add support for the service before doing so. It may even be that Google can't really add support for sending MMS until those carriers are on board.