Take this one with a grain of salt, but iPhone Atlas (who I hadn't heard of before  now) has "received corroborated word that an AT&T employee memo lists MMS ... as a feature of the iPhone 3G."  To iPhone Atlas' credit, they say to take their report with a grain of salt as well, so start shakin' ...

MMS isn't a new technology by any means, but the first-gen iPhone doesn't have it.  Conventional wisdom says that Apple would just as soon have you use their Mobile Me online service, and not MMS messaging, for your photo sharing needs since Apple charges $100/year for Mobile Me.  But AT&T is laying out all kinds of cash to subsidize iPhone 3G's new, lower price, and they'd stand to recoup some of that money by way charging for iPhone MMS messages.

It's worth noting that the new iPhone plan does not include messaging of any sort (the "old" iPhone plan does).  Current AT&T messaging plans start at $5/month for 200 messages.  As with everything else iPhone 3G, all will become clear when the next-gen of Apple's mobile handset goes on sale July 11.