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.