The AT&T Carrier Update (v5.5), required to enable MMS on the iPhone is now live and can be installed via iTunes. Just plug in your iPhone and if it doesn't pop up immediately, hit the check for updates button.

Note: You need to be running iPhone 3.1, so if you haven't updated your firmware yet, do that first.

Once loaded, reboot your iPhone and then head on over the Messages app and you should see the new (for the USA!) camera icon on the bottom left.

For more info, see Apple's support article on the matter, and TiPb's iPhone MMS Walkthrough from earlier today!

Then let us know how iPhone MMS on AT&T is working for you!

Along with iPhone 3.0 Beta 5, which appears to hide away MMS functionality for AT&T, Apple also seeded the second pre-release version of iTunes 8.2, which looks like it blocks editing of the iPhone carrier file. In previous beta releases, developers were able to use the carrier file to enable tethering, something built into iPhone 3.0 by Apple but entirely dependent on as-yet-not-given carrier approval.

Did AT&T ask for the new lockdown? Perhaps the MMS stripping as well? Nothing but speculation so far...

(Thanks Andrew for the tip!)