According to Alex Wiesen, a Googler who works on Voice, Google has been working with carriers and now can support MMS on "nearly 100 different North American carriers". This number includes big names in the U.S. and Canada, like AT&T, Sprint, T-Mobile, Bell Canada, Rogers, and Telus; but, Verizon is notably absent from the list, and testing has shown that MMS still doesn't work between Google Voice and Verizon. This is still a huge update for Google Voice and its users.
Frankly, we're surprised that there are 100 different carriers in North America. That number has to be including much smaller regional carriers that can be hard to keep track of.