I emailed ATT about enabling MMS and they wrote back this...
Thank you for taking the time to e-mail AT&T regarding how to enable MMS
and if we can help you enable MMS because people were able to call AT&T
and enable their MMS. My name is Juliana Heck, and I am happy to help
you with your inquiry.
Mr. Poat, I do apologize but MMS is not available for use on the iPhone
at this time. MMS will not work for your iPhone 3G with the 3.0 software
until AT&T network enhancements have been completed late summer, 2009.
Once the network enhancements are in place, you will be notified via
free text message and MMS will be automatically enabled on your iPhone
3G S.