I think for people in bigger cities that this is the future of how you use a "cell" phone. It won't even really be a phone, it will just be something that has internet access and you make your calls with that. Really here in Dallas you can already do it. It's the same as an office phone, I haven't used a landline for my office phone in years. I use VOIP which I can now use on the go or wherever there's internet, which here in Dallas and most big cities, is everywhere with 3g and 4g.
I actually think that the big cell companies are going to struggle to survive in big cities with tradional service. If you've never made a call over the internet then you should try it. There's no difference at all.