Facetime video chat is currently done at no extra cost for the user as sametime only works with Wi-Fi internet connection.
At least this is how Facetime worked until now. Apple decided to add 3G support after talking with AT&T executives. iOS version 6 will be able to make Facetime video calls through your 3G internet connection. The iPhone from AT&T will come with the latest iOS (right now iOS6 beta 3) which will popup the following message when you try to use Facetime:
“To enable Facetime over cellular on this account, contact AT&T at 611 or visit http://www.att.com/mywireless.”
AT&T already has a powerful 3G connection which can offer the speed and bandwidth required by Facetime. Analysts speculate that this move was done in order to pursue people to use more internet bandwidth and thus sell more expensive data plans.