Apple's FaceTime is currently limited to only being capable of running over a Wi-Fi network, but Romanian Apple website iDevice is reporting that they've noticed new warnings in iOS 5.1.1 that looks to intro FaceTime calls over 3G networks.
It's been noted that running iOS 5.1.1 on an iPhone will be notified that "Disabling 3G may end FaceTime" if they try to disable 3G connectivity during a FaceTime call over a Wi-Fi connection. Putting fuel onto the fire is that iPhones are said to continue FaceTime calls even if the 3G connection is enabled or disabled, which could be a hint from Apple that they could lift the the Wi-Fi-only restrictions of FaceTime.
Even if Apple were to do this, it would then fall on the carriers to take on this feature. FaceTime is considered a Skype-like app, which bypasses traditional voice-minute caps by transferring the communication onto a data plan.