As you might already know, iOS 6 brings this one not-so-startling feature, FaceTime over Cellular Data. For people familiar with the jailbreaking and related stuff, you would know that FaceTime over 3G was already enabled by some jailbreak tweaks for the iPhone.
Apple does copy a lot of functionality from the jailbreaking community but lets's keep that for later.
Here's how to enable FaceTime over 3G (or in other cases, how to disable so that you don't end up using your data allocation):
Note: FaceTime on 3G only works for iPhone 4S, iPhone 5 and iPad 3 running iOS 6.
Step #1. Launch Settings.
Step #2. Tap on FaceTime.
Step #3. Scroll down to where it says “Cellular Data.”
Step #4. Switch it ON/OFF depending on the need.
FaceTime Over 3G/4G: Data Concerns
AT&T requires that you be on one of their Mobile Shared Data plans in order to use FaceTime over 3G/4G. Otherwise, you don't get to use the feature. Verizon, on the other hand, makes it easy for you to access the feature on most of their plans. Sprint is the best of all: it has got unlimited data plans, and it lets you use FaceTime over 3G/4G on any plan that you choose.
Data usage for FaceTime has been variant although it appears that one minute of FaceTime can use up as much as 3MB of data, depending on the type of video quality you make use of.
Data Plans: Sprint Ahead
Sprint is the only carrier that offers a very cost-effective unlimited data plan (with fair-use policy, of course). Unfortunately, though, you won't experience a very strong LTE support in most places. Sprint is only now setting up its 4G connectivity across the U.S, and it is expected that it would be some months before there is widespread coverage.