TracFone (Pay as you go service) users can use:
cellnumber@mms.att.net
Might want to be sure which network the phone setup for first. TracFone uses Verizon, T-Mobile, and AT&T for CONUS operations.
I'm now on StraightTalk, another TracFone/AmericaMovil subsidiary. $45/month for unlimited data/minutes/text on the AT&T network using my iPhone 3GS. (No need for unlocking, it is a rebranded AT&T sim. Just need to jailbreak to change the settings for MMS to work. Too bad you have to spend $200 on one of the Nokia smartphones to do this, they are the only ones with non-phone locked sim cards.)
You can use @mms.att.net for sending pictures and it works, both on AT&T and StraightTalk. I am not sure if AT&T charges them as MMS messages for only text sent to those addresses, so beware of that before you start a multi-hundred part conversation. Might want to stick to @txt.att.net unless you actually want to send photos. FYI, if the person getting a text on AT&T replies it will work and send it to the originator.