The iPhone 3G is a data monster. With speeds of 400, 600 or more than 1400kps reported in some areas, it sucks down information faster than El Jobso does veggie smoothies. Unfortunately, not every carrier in every country provides unlimited data plans to go with Apple's next generation revolutionary internet device, or even reasonable data. And even those that do typically have a "soft cap" (e.g. 5GB) after which they either throttle down your speed, or put a black mark in your record as a problem customer and eventually give you the boot.
If you're in one of the Scandinavian countries, in Mexico, Belgium, or any other Pacific or European, Latin or African country with very expensive data, or if you just want to keep track of how much you're using and when, Apple has provided you and easy way to do it. Read on to find out how!
From the Home screen tap the Settings icon.
In Settings, tap General, then tap Usage. Near the bottom, you'll see Cellular Network Data, both Sent and Received.
WARNING: Your iPhone may not always exactly match the data usage of your carrier, so if you're near your limit, or have any reason to be concerned, always check directly with your carrier for updated, billable data usage.
To reset your usage data (for example, if one month ends and you want to start a fresh count for the next month), just scroll down and hit the Reset button.
As an alternative, you can consider using it like a car's odometer instead, and just note down where you are at the end of one month, and subtract that a the end of the next to get each month's total, and an overall total.