Last Friday, AT&T added multimedia messaging (MMS) capability to the iPhone 3G and 3Gs, allowing customers to send and receive video, pictures and audio messages in the same way they could send text messages.
To enable MMS messaging, connect your iPhone to your computer and click “Check for Update” in iTunes. Once the update is completed, restart your iPhone by turning it off then on again. Note that iPhone OS 3.1 is required so you may need to perform that software update first. Once you connect to iTunes, you will be prompted through the correct steps.
I've been able to successfully test this within the AT&T network, but still cannot send or receive MMS messages across carriers. Can anyone else confirm if this is working or not?