The vast majority of people that will be using video conferencing surely won't be doing it walking down a street. They will be sitting in an espresso place or some other place that has WIFI so 3G won't come into play for that in most circumstances.
Battery life will be about the same as it is now if you're using WIFI.
I don't see the reason to do video conferencing in a coffee place. U want to be able to show the other party what u are looking at. Like sightseeing in Miami or Paris or from the top of the Empire State bldg... Or from home talking to your girlfriend and having a naughty video conference...