AT&T ended its unlimited data plans in 2010, and Verizon followed suit in 2011 just a few months after the iPhone landed on its network. Meanwhile, T-Mobile's "unlimited" data is throttled -- or slowed way down -- after you've used a couple of gigabytes on its network. So unless you're willing to go contract-free (and iPhone-free) Sprint's $79 a month offering is the only truly unlimited data plan left that's open to new subscribers.
(A recent report suggested that Sprint was throttling data for its most excessive users, but was in error. The throttling only applies to people who are roaming.)
The question is, do you really need an unlimited plan, or would you be better off with a cheaper option? Is there any way to tell?
Estimating your data usage
There are a number of apps and online tools that will let you do this. The newest version of Android, Ice Cream Sandwich, even comes with a built-in app that not only shows you how much data you're using, it breaks it down by which app uses the most on which days. Many other smartphones keep a running tally of how many megabytes you use up.
Verizon's online calculator lets you get a ballpark estimate ... if you can accurately remember just how many emails you send in a month. However, Marguerite Reardon of CNet summarized how much data each activity counts for in her "Ask Maggie" column. The upshot? Emailing is basically free, while web browsing can add up but only if you do it a ton.
What really racks up the data use
The biggest culprits are streaming music and videos. If you've got all your songs on Google Music, say, and you listen to them from it instead of downloading them, you've got about 50 hours you can do that in a month before going over the limit for AT&T's $25 (for 2 GB) plan ... and that's if you don't do anything else.
How about videos? That same plan will let you or your child watch one twenty-four minute episode of Avatar: The Last Airbender per day on Netflix, in low quality, for every school day in the month. Alternatively, it'd let you stream about two and a half HD movies before running out.
So as you can see, it's the music and video that eat through your data allotment, especially HD video.
Are you "that person?"
According to the Nielsen Company, as of 2010, "the top 6 percent of smart phone users are consuming half of all data." In other words, most people don't watch Netflix in HD on their daily commute on the train. But a handful of people do things like that, and they're possibly the only ones who "need" unlimited data.
If you're not watching videos over 3G every day, or listening to Google Music every hour that you're at work, you'd probably do better to measure your monthly data usage and then find a tiered plan that's right for you. Preferably one that won't penalize you too bad if you go over, just in case.