I can confirm that the following steps will allow Swirly to work on your AT&T iPhone. I got this from the swirly forums and it worked for me.I think you need family plan with messaging on it also, which I do.
First, you will need a regular AT&T/Cingular phone. I used a old Nokia 2610. You should be able to use any phone as long as it is Media Net compatible.
Take the SIM from your iPhone, Insert it in to a standard phone, Make a phone call, wait about 5 minutes.
After 5 minutes go to the following link,
http://mymedianet.wireless.att.com/app/MC/Default.aspx
Enter in your mobile number(it might say your phone is not supported.. if so wait a little longer and try again), then at the next screen it will ask for a 4 digit code.
The 4 digit code will be sent to your phone via Text message.
Enter the code in on the website and then you should be able to access the main Media Net site.
Now send a mms. It will take some time for the network to recognize the phone. Be patient. Took me about 10 minutes. Dont continue until you have successfully sent a message.
Now take out the sim card and place it back in the iPhone.
Make sure the Swirly MMS Settings are as follows...
MMSC: mmsc.cingular.com
Proxy: 066.209.011.032
leave apn, username, and password fields blank..
Image Size: Maximum
UserAgent: Nokia_N95.