AT&T may be preparing to charge customers to use the FaceTime over cellular feature found in iOS 6. Preliminary testing with a developer version of Apple's next major software revision results in a popup message prompting the user to contact AT&T to enable the new feature.
Up until iOS 6, FaceTime has only be available over Wi-Fi networks, presumably because it eats up a lot of data and / or 3G networks simply weren't able to provide enough bandwidth for video calls. The new iPhone is expected to carry a 4G radio which would eliminate the latter concern for most customers.
AT&T reached out to 9to5Mac, saying they are working closely with Apple on the new developer build of iOS 6 and that they would share more information with customers as it becomes available. Notice how they didn't actually deny the idea of charging to use the feature?
Apple also notes on the iOS 6 preview page that carrier data charges may apply and that the service isn't available in all countries. FaceTime over cellular will require an iPhone 4S (or newer) or third generation iPad.
Of course, this wouldn't be the first time that AT&T has charged for extra features. The company requires a fee to use Personal Hotspots, a function that allows multiple people to share an Internet connection from a single device. The going rate for that feature on the iPhone is $50.00 for 5GB of data - hardly worth the price by most users' standards.