I've logged into my account, and clicked around everywhere, and can't seem to find this. Is there a way, on the AT&T Web site, to see how many rollover minutes you have?
The iPhone has a fantastic feature built-in. Navigate to 'Settings' > 'Phone' > ATT Services and they will text you with current minutes.
Also if your billing cycle just ended it may take a day or two before they appear. But you will see them.