I think the only way you will see unlimited data return is IF AT&T and Verizon start seeing a lot of churn.
If Sprint keeps unlimited - a big IF,mthen I think as contracts expire, you will see some customers jumping ship. Also, with Straight Talk and the other MVNOs out there, customers have more options.
Let's face it, the Only reason sprint pushed unlimited data was because until they got the iPhone, they were hemhorraging customers. Their service isn't as good as AT&T or Verizon. But people are still willing to put up with that to get unlimited data and potentially cheaper monthly bills.
Also, the iPhone is not very profitable for the carriers at the current subsidy level. There has been talk that the subsidy may decrease, meaning people will have to pay more for the iPhone.
If this happens and enough people are frustrated with the big two US carriers and they decide to jump ship - we MAY see unlimited data used as the carrot to get people back.
That is assuming a lot, however, and once the carriers start charging for something, rarely do they give it back for free.
Unfortunately, it's similar to gas prices in another way. A month ago, gas was over $4/gallon here in the US but people still filled up their vehicles. We are a society that is dependent on our cars. We are also dependent on our cell phones. The best way to protest the loss of unlimited data is to go back to a flip phone so that the carrier loses that additional RPU. But who can go back now? We're just going to keep paying what they charge us and get less and less in return.
I was actually encouraged when Verizon's customers protested the bill pay fee. Too bad we can't energize enough people to do the same for unlimited data.