I don't think anyone needs a crystal ball to know this, but I fully expect AT&T, Verizon Wireless and others in the USA to join in on what is going to be a when, not an if. It's about paying for calling over mobile data networks vs. the free ride we now have. Look at what just happened in South Korea. This will only happen more as LTE rolls out, as the quality of calls on LTE is vastly better than over first generation GSM 3G or CDMA, and markedly better than on HSPA+ networks due to spectrum allocation and better handsets and tablets.
We already know about TeliaSonera's efforts, the blocking of VoIP and Skype on Yoigo in Spain and the way Skype is blocked on GoGo flights in the air. The technology is out there, and now operators are saying, "you can talk, just pay for that" as the call revenue is erroding away from their overall minutes.
This is clearly, the shape of things to come and I don't see anything wrong with it. Just do the math, see if you really save money, make more use of WiFi and you'll end up ahead....don't do the math, burn up your data plan and avoid WiFi and you'll see how much you lose.