Pray that this mindset does not infect your ISPs and tempt them to introduce traffic caps. Having your users be paranoid that they're going to go over their cap and get absolutely reamed for overage charges is not exactly how you build an advanced, high-bandwidth content delivery system. Even less are going to be "data pigs" that use a trivial 1.5GB after such a change, high prices guarantee that. Silly buggers.
I don't think this would be such a bad thing -- just as you pay on a per-unit basis for the water and energy you use, paying per unit of bandwidth consumed seems entirely reasonable. In an unlimited plan, typical users effectively subsidize a small minority of heavy users.
Of course, the real problem is the outrageous charges for when the bandwidth cap is exceeded, as well as the lack of proper notification for when you run over your limit.