When FaceTime was first introduced, people often wondered if they would one day have the ability to make calls over 3G. At first it was only available on Wi-Fi, but no longer.
As part of the announcement for the new iOS 6 mobile operating software, Apple said that its users will now be able to take and receive FaceTime calls over a cellular network. (3G for iPhone, 4G for iPad) This is a huge deal for FaceTime users, as it makes video calling truly mobile.
Carriers may have to deal with a lot of extra data through their networks, which was though to be the reason Facetime wasn't cellular network enabled from the get-go. If you have unlimited data, then go for it. Otherwise you may find your data cap has been maxed out after just a few calls. But, still, it's nie to have the option.
Apple will also unify your Apple ID and your phone number, allowing you to take calls from your Mac or iPad with FaceTime.
Apple is constantly trying to merge its mobile and desktop/laptop devices while trying to keep you constantly connected to an Apple machine of some type. This will allow people to take a call from their iPhone and transfer it to their Mac if they happen to be at their computer.[TNW]