Here's the scoop: the Data Meter on the iPhone is very helpful for letting you know how much data you've used, as Rene pointed out a few weeks ago. Sadly: it's not a "Cellular Network Data" meter proper, apparently. Tsopanos has figured out that if you play around with the Access Point Name (that's APN to the rest of us, and no, AT&T owners won't see that setting under General -> Network), you can get that meter to return false information on 2.0.2.
More specifically, if you 'break' your APN by changing or screwing around with those settings and then go back and browse any data you have cached on your browser, email, or maps, the iPhone believes all that is more data that's been downloaded over the cellular network. What this means is that the Cellular Data Meter might actually be double-dipping when your APN is right, i.e. counting locally cached data as cellular data. We're not sure yet as we don't have an iPhone with editable-APN sitting around -- browsing cached data while in Airplane mode doesn't seem to register.
Yeah, it's a esoteric bug and yeah, at least that bug errs on the side of caution by showing you potentially more data than you've actually downloaded. Still, something we're hoping will get a fix.