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