Unlimited will not be coming back....ever. I don't work for AT&T but I used to. The fact is, carriers are no longer making the same profits they used to. Clearly I can't make this statement without explaining so here goes.
Carriers always have different ways to make money, when cell phones first came out, it was based off minutes, network time was expensive, and costs needed to match that. As prices for network time/bandwidth dropped the prices stayed the same. Increase in profit with a decrease in operating cost.
The next step in profits was text messages. At first they used extra network time as well, an increase in cost, 1kb each, not too bad. Carriers soon figured out how to send text messages when the phone checks in with the tower decreasing cost. Text messaging was a big cash cow back in the day.
Clearly next up was Multimedia Messages, since data was in it's infancy, Multimedia messages were included in separate packages from text so profit was maintained. There were some plans that were unlimited because carriers were making a significant profit off of texting still. Unlimited plans back then actually were unlimited because web pages didn't take much data and mobile web was in it's infancy with no real streaming back then.
Since then there hasn't been too many innovations, and carriers tried to find different add-ons to increase their profit. At first this worked, with Tele-Nav but as smartphones started to include these services for free (built in or downloaded application) their profits have shrunk. People also aren't texting as much as they used to, cutting back on their text message plans and using data instead. While the Data really doesn't cost the carriers money, they aren't making the same profit as they used to. Logically the next step to make money is to charge for Data. They clearly know their current data restrictions are too low, but at the current levels they are making a profit. Data limits will continue to increase but never will be truly unlimited again not in the way we used to have them, because the carriers can't make their profits.
An ideal situation would be you pay for 3GB (or whatever) Full Speed, and once you hit 3GB you get throttled, not to the level AT&T is currently throttling, but say 1/3 speed or whatever is equivalent, leading to people who really want the speed to pay for it. Alas a plan like that is probably a good 3-4 years out, once LTE is rolled out much wider. At that point Data costs will have dropped even further and these plans would be an option.