If you don't know what a widget is, your a goon. Open your dashboard by hitting F12 (or F4 on newer mac's) if that doesn't work, panic!
Get yourself the iSlayer istat pro widget
Now, it monitors network interfaces, pretty wild eh? When you connect your iPhone by USB or Bluetooth, you should see it as an interface in istat... keep your eyes on that data if your monitoring your usage. OR use your providers usage page, OR, check out iiQuota on the appstore (supports most of the big name carriers)
Enjoy!
