As promised, AT&T's iPhone finally has the capability to send multimedia messages to another phone number. Whatever AT&T had to do to gets its act together, the carrier did it.
Use MMS on your iPhone
You'll need to connect to iTunes to receive the update, which should download in seconds. We were able to send an update immediately, but you may have to restart your iPhone first.
To confirm that your device is ready, go to Settings > General > About. Your carrier version should be "AT&T 5.5," and you'll need the latest iPhone 3.1 software update. Also, remember that owners of the original iPhone will miss out on the fun.
For more information, check out Apple's links for updating and enabling carrier settings. And if you're having problems, see Apple's troubleshooting tips or the MMS send failure fix from iPhone Atlas.
Welcome, AT&T iPhone, to 2003. Now we just need that tethering.