AT&T's been feeding us a story that it'll offer MMS on the iPhone only "once [it completes] some system upgrades that will ensure our customers have the best experience," but here's the thing: it seems that it works right now -- if you've got a build of OS 3.0 that'll let it. None of us have been able to get it going ourselves, but we've been sent screenshots from a tipster that seem to indicate that both MMS and tethering are in full effect on the network, so there doesn't seem to be a network restriction involved (we can't verify whether the build we see here was tweaked, so we're thinking there might be some hackery in play). The strategy for ultimately deploying the features to AT&T customers is unclear, but seeing how some folks in Europe running the exact same gold build of 3.0 have access to MMS and those of us stateside do not, it's got to be a software switch that can be triggered remotely. Some form of SMS provisioning, perhaps? Let us know what you've seen out in the field in comments.
Update: Turns out the secret to enabling MMS and tethering lies buried in the carrier settings file, which happens to be the same black magic employed to roll out 3G MicroCell support a while back. Customer devices are at AT&T 4.0, but there's an AT&T 5.0 profile floating around the interwebs that turns the features on -- it's just a matter of finding it, installing it, and making sure that your account is provisioned for multimedia messaging. Thanks, everyone!