The system is already starting to work. I received a Facetime call on my ATT Iphone 5 when I was walking on the street in NYC about 10 days ago. It was pretty stable. I haven't seen a big jump in my data use from it so it is probably not too bad if you use it occasionally. It sure beats having to be on wifi to receive a call.
Maybe there aren't enough factories in the world qualified to make Apple products. If the Iphone and the Ipad draw too many manufacturing resources to meet demand then the Mac computer line may be stuck with second string. Same might be true of Apple engineering systems.