According to BGR, the reason AT&T isn't going to be ready with MMS at launch is not because of any new pricing plan—it's because of Opt-Out MMS codes.
Apparently, because people were using MMS with the iPhone 3.0 betas, AT&T put an opt-out on all iPhone accounts. So, theoretically, if AT&T manually (yes...manually) removes all of these opt-outs, MMS would work just fine.
As for the tethering delay, BGR also noted that AT&T is still in the process of hammering out a pricing plan—possibly as much as $70 per month for unlimited data/tethering. None of this is confirmed of course, but $70 a month for an unlimited data plan with tethering would be way to damn much. [BGR]