AT&T has revised its unlimited data plans for iphones and have created a tiered data plans in which users can select the dadata plans based on the monthly usage 
here are the revised AT&T’s plans
$15/month for the DataPlus plan which includes 200MB of data transfer. If customers exceed 200MB in a monthly billing cycle, they receive an additional 200MB of data for $15 until the end of that billing cycle.
$25/month for a 2GB data plan. If subscribers exceed 2GB during a billing cycle, they will be charged $10 for an additional 1GB of data.
Most of us are confused about which will plan will be economical.For thats its better to check your usages so far and choose a correct one
Steps to find your data usage on iphone
- Go to AT&T’s website http://wireless.att.com/ and login with your account.
- Choose Bill & Payments.
- Now Click Create a Billing Report. It’s under the Billing Reports heading.
- Choose Data Usage Trend from the Report pop-up menu. You can also set a date range to see your data usage for different time periods.
The Usage report will show you , how much data you have used per month (uploads,downloads).
Iphone App
You may find the data usage using the iphone app also- AT&T’s iphone app
The New iphones and ipads will also be having this new data plan.Its better to check out the usage trend to select the plan.
You can also use AT&T’s data usage estimator web application to view the data usage based on your requirement 
