Same here. Wife and I both have 4S models. OS 5.1 is causing massive data usage according to carrier, yet iPhone's own Cellular Data usage numbers are normal. There's a massive discrepancy between Apple's report of cellular data use, and the carrier. iOS reports 600MB used in about 6 months, which is about normal. Carrier reports 750MB in just the first 6 days of this billing cycle. Wife does not have Exchange or ActiveSync.
What I find most odd is that whatever AT&T is seeing for data use, the iPhone isn't seeing it the same way. It's not even close.
Same problem here. I have 5 phones on my account and here's the data from the last month.
Usage* AT&T % Difference
_______ _______ -------------------
iPhone 4S 858MB 3,508MB 308%
iPhone 4 178MB 229MB 29%
iPhone 4 72MB 72MB 0%
iPhone 4S 114MB 1,705MB 1395%
iPhone 3G 122MB 266MB 118%
AT&T data group is clearly denying this, claiming that they can't confirm that what the phone reports is consistent with what AT&T sees. Note that a different group in AT&T did speak with Apple and confirmed that usage on the phone via *Settings, General, Usage, Cellular Data Usage (reset at the beginning of the billing period) should match AT&T's systems.
We need someone from AT&T to dig in to this and confirm whether their data collection is accurate.