i would only worry about data usage if you are on the $15/200mb plan. Most people do not realize 2gb of data is way more than enough. I thought I used a lot of date but i only used close to 1gb on my Aria last month
Edit: AT&T also sends you free text messages when you are at 60%, 90%, and 100% usage
Edit one more time: AT&T does also have a "my wireless" app which allows you to view your account from your phone
and just as an idea, for this bill cycle (15 days into the cycle) I have used 548mb of data, so if I keep using as much data as I do (and I work at AT&T so I use my phone a lot) ill only hit a little more than 1gb