Are you worried that he might access all your data and information on that phone? If yes then don't panic. I believe you must have set up pass code to access the home screen. There is no way that he could access your home screen without performing hard reset of that phone unless somehow that douchebag managed to guess your passcode. Once, he goes for hard reset then all your data on your phone would be erased from that phone. 
ATT can track and black list the phone but I don't think they do that often. So, dont pin hope on ATT.

Update: Seems like there is a way to bypass passcode on iphone but not sure which version of IOS is there on your lost phone.