Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_1 like Mac OS X; en-us) AppleWebKit/532.9 (KHTML, like Gecko) Version/4.0.5 Mobile/8B117 Safari/6531.22.7)
There's the myWireless app from AT&T that allows u to monitor all Ur usage
where are you located? in the US, at&t has the "my wireless" app. the iphone itself also has a usage counter in the general settings. you just have to manually reset it every billing period.
Check out DataMan on the App Store. It allows you to monitor your data consumption in real time and gives you push notifications when you hit data thresholds set by you.