This is happening all around Europe as we speak,

Smartphone users are consuming such an amount of data that the flat-fee subscriptions are not enough to cover the costs any longer. Operators will change to basic subscriptions with limitations and bolt-on offerings for additional traffic. You can run away from T-Mobile now but the others will adopt until there is no where to go to.

Check out Layar.com for a cool Android application that has the exact same vision. Their video on the site shows a demo with houses for sale in Amsterdam.