Griffin Technologies’ “Griffin blog” has a great tip from one of their readers.

How to check your iPhone data usage

The tip is to dial *3282# which results in a text message from AT&T with your current data usage and the date of the start of your next billing cycle. You can also get this information when logged in to your AT&T account or from AT&T’s free myWireless app.

Based on my current data usage rate, it looks like I’m going to use between 500MB and 600MB of data during the current billing period.