I am posting this because AT&T does not make it easy to report fraudulent accounts.
My situation
I started receiving emails from AT&T clearly stating that an AT&T Wireless account had been created using my email address. I got everything new subscribers get, including welcome messages, account verification messages, paperless billing setup messages, and new bill notifications.
What to do
Don’t bother phoning AT&T. You will be referred by various call center representatives. None can do anything. Instead, go directly to the Mobility Fraud Inquiry Form (link current at time of writing).
Tips
You do not have to provide any personal information other than the fraudulently used email address, the last four digits of the fraudulently obtained AT&T phone number, and the fraudulent account number. AT&T eventually tells you the last two during a monthly billing cycle in the emails it sends. I put obviously bogus information for my real name, address, SSN, zip code, etc. (e.g. n/a, 000-00-0000, 01234).
My experience
I received no response to my initial report. But when I started filing a report every time an account-related email appeared in my inbox, AT&T responded quickly. He canceled the account and sent me official confirmation that the account was fraudulent and should not have been associated with my email address.