FaceTime – the video calling feature used on the iPad, iPhone, and Mac – will not work on the new iPad over an LTE connection. It will still require a WiFi connection, just as the current iPad 2 does.
The Verge’s Dieter Bohn has tested this out and confirmed (along with a screenshot) that FaceTime prompts for a WiFi connection.
Before you get too angry with Apple about this situation, bear in mind that the consensus speculation says this is very likely to be something that cellular carriers are insisting on, rather than an Apple restriction. So there’s hope at least that perhaps one of the carriers will re-think on this policy at some stage.