Skype already works on 2G/3G/4G. Skype video chat has been available for smartphones on wireless carriers for over 2 years. Data usage on my iPhone averages about.... 100MB or so per hour. When in the US, I have a 3GB package. I used to use Skype maybe 10 hours a month over 3G. Perhaps my usage patterns aren't very heavy, but I'm usually near Wifi, so I've never gone over, usually end up with about 2.5G per month or so.
Addendum: Data usage actually varies a bit. On most calls, I hover around 150kbps, but occasionally, I connect to HD, at which consumes data much faster, I think about 5X faster. When that happens I ask the person on the other end to dial it down and call me back.
EDIT: The data usage refers to video calls. Voice calls use a lot less.