Starting with iOS 6, everyone with an iPhone will be able to have a FaceTime video chat with other iPhone users or Mac users. No WiFi Needed - it works over 3G. We just did some tests at 3Gstore.com and found that FaceTime uses around 2.42mb of data for 1 min of conversation. That is around 145mb an hour or if you area allowed 2GB/month, you can do around 13 hours of FaceTiming a month and nothing else. So while it will be cool and it will work great - keep your FaceTime over 3G conversations at a minimum, or else you will blow through you monthly allowed data usage.
For more info see: What does 2GB Get Me?
