With the new version of Skype we are allowed to perform video calls from the iPhone on both Wifi and 3G. But obviously we must take into account the bandwidth consumption that we will have when using the application in 3G, since if we have a limit with our operator, we will not want get over it.
The web 9to5Mac You have already carried out the test: you have made a one-minute 3G call and the result has been a consumption of 3.5 megs per minute roughly, which is a high number. In total 1.3 megabytes were sent and 2.1 were received.
If you have contracted an unlimited internet connection, then you should not worry about data consumption. Yes indeed, Safety that the telephone operators will attack this option of Skype due to the considerable increase in the consumption of data packets.