This has been a long time coming. Last year, AT&T and Verizon Wireless began forcing new users into tiered (limited) data plans, with a certain amount of data allowed per month. The typical allotments are 500MB, 2GB, 5GB, 10GB, etc. The price of the plan rises with the amount allowed. Meanwhile, old users are supposedly "grandfathered" into the unlimited data they signed up for. Apparently, AT&T has decided that there needs to be a certain amount of pressure to steer people in the direction of tiered data plans. I haven't heard of anyone having this experience with Verizon Wireless.
As a side note, I find it interesting that people use that much data on their phones. As an example, I consider myself to be a smartphone power user, and my cell data usage is somewhere between 500M and 1GB a month.
On one hand, I think a lot of new smartphone owners are prone to much higher cell data usage than they actually need, because they don't bother to set their phones up to use their own WiFi connection at home. Going by my own experience, I've had 3 users in the last 2 days tell me they didn't even know that was possible, let alone preferential. At the risk of sounding like I am inciting a holy war between Android and iPhone (trust me, plenty of others have done this for me, thank you very much), I'll also point out that they were all iPhone users.
On the other hand, in fairness, I don't have 4G coverage, and I don't watch Netflix on my phone when I am not at home. Also, after some quick research, most of the major data consumers do appear to be Android users, and that makes sense when you consider that the iPhone doesn't have access to real 4G networks. I got a chance to play around with 4G the other day when I was out of Vermont (see my previous post about Vermonters being Verizon Wireless second-class citizens), and my first thought was, "Wow, look how quickly I rack up some serious data usage!" Not having to wait for anything removes all of the subconscious internal limiters I have about using certain functions (or, put simply: my phone is more fun to use!)
I still think there are a lot of people using cell data when they don't need to, and as smartphones become more and more accessible, this issue will become more of a trend.