I have a similar issue - my wife got a text today saying that she is almost over the limit for the 200mb plan, only 3 days into the month! She's previously never used more than 80mb in any month in the last 3 years. What's more, she reset the phone usage statistics on the first day of the billing cycle, and they're showing 3mb up and 8mb down. The key lines on the bill are below. The phones are iPhone 4s, not jailbroken.
Similarly, my account (never over 150mb in any month) has also gone to 95mb, but my phone reports 36mb up and 7mb down.
What's going on here? Does GPS usage not show up on the iPhone's usage statistics? Even if that's the case somehow, I'm at a loss to see how that could explain the bizarre amounts on my wife's account - she really has barely used the phone in the last few days.
09/15 08:49 PM phone Internet/MEdia Net Sent 91847KB
09/15 08:44 PM phone Internet/MEdia Net Sent 54KB
09/15 08:28 PM phone Internet/MEdia Net Sent 2271KB
09/15 05:52 PM phone Internet/MEdia Net Sent 1548KB
09/15 04:23 PM phone Internet/MEdia Net Sent 1198KB
09/15 11:44 AM phone Internet/MEdia Net Sent 650KB
09/15 10:41 AM phone Internet/MEdia Net Sent 1510KB
09/15 01:01 AM phone Internet/MEdia Net Sent 7854KB
09/14 07:17 PM phone Internet/MEdia Net Sent 38849KB
09/14 02:30 PM phone Internet/MEdia Net Sent 30136KB