I dont think it can use your minutes. It's WIFI only, so you could be using your own internet connection at home, so why should your carrier be allowed to charge you for using it?
When you start with a phone call and then switch to facetime, It must seamlessly disconnect the call through the phone network and switch to using WIFI.
Personally, I think its a bit rich for Apple to be making this out as so revolutionary as they do in the video from the end of the keynote. Video calling over the carrier's network has been around for years! I think in the UK it came around 2004 with the first 3G network. Apple isn't even using 3G. However this never took off, Loads of phones are now capable of video calling but no one uses it. If anything, Facetime has the best chance to make video calling popular