Most of us iPhone old-timers in the US were grandfathered in to AT&T's Unlimited Data Plans. However, for new subscribers those data plans are gone. You now have to choose between a 200MB/month ($15/month) or 2GB/month data plan ($25/month). Of course if you're on a limited data plan here in the states or abroad you're going to also want to know when you're reaching your limits. There are a few different ways of checking your data use if you're on AT&T. You can *3282# from your iPhone and you'll get back an SMS displaying your current month's data usage. AT&T also provides an App to check your account information and usage. While these methods work, they require you to actually remember to check manually. DataMan is a neat little App that provides you more real-time data usage information while running in the background under iOS 4. Best of all you can set your own usage monitors and alarms. You can set a daily, weekly and monthly threshold for how much data you want to use and then be alerted to the fact that you're about to go over or have gone over your own preset allowance. The App does something that the AT&T methods don't and that is it not only tracks both 3G AND WiFi usage, but it even geotags where you were using the data. This key for travelers that want to data roam.
Do I need unlimited data?
If you're on an unlimited plan it's probably easy for you to dismiss this App. However, this app will help you answer the question "do I need unlimited data?" AT&T does offer a cheaper DataPlus plan that's limited to 200MB of data a month. I would dare say that there are a lot of people out there that could get by on this plan and save money. With DataMan you could track your usage on both 3G and WiFi for a couple of months to determine whether or not you really use more than 200MB or 2GB's of data each month?
A couple of Warnings! This App REQUIRES iOS 4! So if you're not running iOS 4 it may not even show up in the App Store as a download for you. Secondly, the App DOES need to run in the background in order to track your usage. You will notice that the Location indicator in the upper right corner is ALWAYS active. I asked the developer about battery life impact running Location Services all the time and he assured me that his App was using the new low power option in iOS 4 that only checks the location when you change cell towers. He claims to easily 30 hours of battery life between charges. I too have not seen any significant change in battery life on my iPhone 4 since running the App. YMMV!
You can get DataMan here from the 
