I have been working of rseveral days now to get my iPhone working with AT&T, and I believe I have found out why there are so many different "real solutions" floating around out there. I tried them all and was never able to get mine to work, and after several calls to AT&T tech support, I finally told one of the techs exactly what I had done (jailbreaked the phone, and changed the IMEA number to one from an old RAZR I had). He suggested that I go to the device tutorials listed on AT&T's website under "Support" and try out some of the settings in there listed for the different devices. Apparrently, there are different MMCS/proxy settings that AT&T uses, depending on what phone they think you are using, and some of them will work with a hacked iPHone, and some won't. Even the ones that might work with one person's iPhone will not necessarily work with another's, depending on how AT&T has your individual account set up.
I did this by going to the website (don't log in to "My Account"), clicking on "Support", and then selecting a phone uder the "Phone/Device Support" section. Click on "Tutorials", and then select "picture Messages" under category, and "Configure MMS Settings" under topic. Some phones will show the same settings as what is already listed in this forum, and some will be different. If you try one, and it doesn't work, select a different phone model, and repeat the tutorial. You'll get a different setting to try. Mine ended up working with the settings for an AT&T model 8525. One thing you should understand is that what SwirlyMMS calls the "MMSC" setting is referred to as the "Server Address" by AT&T, and what SwirlyMMS calls "Proxy" is referred to as the "Gateway" by AT&T. Also, if the AT@T settings call for a port number, then it needs to be inserted into the "Proxy" settings by typing a colon (:) immediately after the IP address, and then typing in the port number.
What I am using on my phone now are the following settings:
MMSC: http://mmsc.cingular.com
Proxy: wireless.cingular.com:80
APN: wap.cingular
no user name, no password
Again, those settings work on my phone because that's how AT&T set up my account after I told them I quit using the iPhone, and went back to my old RAZR. If you try the same thing, your account may require different settings, depending on how they handle the change, but I do believe that one of the settings listed in the device tutorials should ultimately work for you. Good Luck!