AT&T iPhone customers can view their data usage for the current billing period on the summary page for their account by logging in to the AT&T Wireless website.

Unfortunately with the latter you're restricted to viewing your usage from the current billing cycle, but maybe, just maybe, the app will get an update. How much data do you use a month?

You can view your Wi-Fi data usage in the same way as cellular usage. Figure E Image: Megan Crouse/TechRepublic Now you’re able to view and control Wi-Fi data usage on your Android device.