The FaceTime app lets you do video chatting between Apple devices but some have felt it was limited because you had to be on WiFi. During the WWDC 2012 keynote, Apple said that will no longer be the case once iOS 6 comes out. People will now be able to reach you via the phone call or with a direct FaceTime call, nearly anywhere you are.
Video calling over mobile networks isn’t anything new but it is nice to have it baked into the iPhone natively. Of course, you have to wonder how much data this will take off and how quickly FaceTime-ing with your friends will blow through your data limit.