Unlimited smartphone data plans were never sustainable. As data use outstrips voice, as smartphones get ever more capable, and as the Internet introduces more data-intensive applications, it became clear that wireless carriers could simply not afford unlimited plans. High-def video streaming from Netflix and Hulu are on the way, and two-way video chat is already here (on the HTC Evo 4G), for example.
So tiered data plans were a given. Verizon explained the "buckets" tactic the company plans to take  with the rollout of its 4G network next year, in which customers purchase a set amount of data in chunks. But it's AT&T that's the first to announce  a tiered data service. Formerly, they offered a $30 unlimited data plan, and if you're already on that, you're welcome to keep it. But some may actually want to drop it to move to a lower tier.
AT&T's data plans now include two tiers, plus an add-on feature, and, thankfully, there are several safeguards to ensure that customers don't go wildly over their allotted data. These customers will receive warning texts and emails after they hit 65%, 90%, and 100% of their monthly allotment, and if they go over, they'll simply pay a small amount to buy another "bucket" of data.
The first tier, called DataPlus, includes 200MB of data for $15 per month. That's actually good news for, say, BlackBerry users, whose phones are great for email but not so great for data-intensive tasks like web video. If these users exceed that 200MB, they'll be charged another $15 for another 200MB. AT&T, interestingly, estimates that 65% of its smartphone customers use less than 200MB of data per month.
The second tier, DataPro, bumps the limit to 2GB, for $25 per month. If a DataPro customer goes over the limit, they'll have to buy extra "buckets" of data at $10 per 1GB of extra data. Tethering is only available for DataPro users, and it'll cost $20 per month.
As far as the iPad goes, existing iPad owners can keep their $30 per month unlimited data plan or switch to a new $25 per month, 2GB plan, like DataPro except without a contract. New iPad users have to use the 2GB plan.
As loath as I am to praise a wireless carrier, these tiers are actually not the death knell of data we all thought they'd be. For some, it'll mean a lower bill, and for practically everyone, it won't make much difference.
But that won't change the fact that it's the power users who will be affected--and it's the power users that are the most vocal about changes of this sort. Though current AT&T users can stick to their unlimited plans, a new user who wants to use 5GB of data per month is going to find themselves with a very pricey plan.
Of course, I expect both tiers will increase their allotment as time goes by--let's just hope it keeps up with smartphone features.