I had one on my Android that you would put your start and end date in, and your limit, and it would track your data usage and estimate when you were going to hit your limit. It's a nice app for keeping track of usage if you have a data cap.
Apps can't run in the background so something like SPB Wireless Monitor won't work. But the AT&T app works well. Check your carrier app hub and see if they offer something.
On at&t, you can dial *DATA#, and you will get a text message with your data usage. This works from any at&t phone. Other shortcuts include *MIN# (Remaining Minutes), *BAL# (Payment Owed)