In the last couple of months, ATT has had many issues with identity theft. Typically, the thief would call up on behalf of another company, have ATT ship a phone, and then start running up a nice tab. The interesting thing is that the phone is not usually sent to the thief directly. The phone is usually sent to the company who is being exploited. Then, the thief just uses an existing cell phone to correlate the phone number to. It is very easy to associate a phone number with a cell phone. You can just call up ATT or sometimes go online if all the info is handy. 
How to prevent cell phone identity theft?
Set up a password with your mobile phone provider. This will ensure just you or anyone who you gave the password to will be able to order or make changes to your account and orders. This is the #1 way you can prevent others from racking up phones, minutes, and time on your dime.