This peaked my interest so I looked into my own data usage and usage history.
I've been on unlimited data since the iPhone (E) through iPhone 3g (3g), Nexus One (3g), Atrix (4g/hspa+) and now the SkyRocket (4g/lte) and on very few occasions have I even broken 2GB a month.
ATT's website only allows me to go back through my data to May 2010. On average over the 18 months from May '10 - October '11 I used 1.17GB/month. Only one month did I go over 2GB (Nov '10 = 2.1 GB). There was no noticeable difference in usage from Nexus One to Atrix (3g to 4g). It wasn't until you look at the usage from Oct '11 to Nov '11 that you see a difference. In the month that I've had the SkyRocket I have supposedly used 3.7GB of data and 778 MB in the past 7 days. The 778MB puts me on track to use another 3.4GB this month...2.9X my normal/historical usage.
As many of you have mentioned, I too have not changed anything regarding my usage patterns. I have the same accounts set up, widgets are set to check on the same frequency, etc, etc, etc.
I haven't received a data overage warning but this does seem a little suspect to me. While checking my usage history I checked the other phones on our account and noticed that my wife used over 6GB one month and she never received a warning message. This begs to question what is the trigger that prompts the warning message.
I posted over on the other AT&T LTE page (HTC Vivid) to see if any of them have gotten the message or noticed abnormal additional data usage.
Data Overages
I'm going to keep a close eye on my usage and will be calling AT&T every couple of days to see what is going on.