As a follow-up to Thursdays announcement that multimedia messaging is finally arriving for iPhones on Sept. 25, AT&T has posted a video outlining the growth of smartphone traffic to explain why its taken this long to deliver MMS. The video stars Seth Bloom, AT&Ts spokesman, who explains some of the technical intricacies involved in expanding the network to support services, including MMS, for iPhone. Here at Wired.com weve corresponded with Bloom often for comment regarding AT&T and