According to CNN

Skype released a new version of the telephony application for iPhone, which adds video-chat features that go beyond what Apple offers. With the Skype app released on Wednesday, owners of the iPhone 4 and the older 3GS model can make video calls over AT&T's 3G network.

Apple's own FaceTime video-chat feature, which comes with the iPhone 4 and new iPod Touch gadgets, cannot make calls over AT&T's network, only on Wi-Fi.  Apple said carriers requested time to upgrade their networks to handle the traffic load of video calling.

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