I've been a iPhone user since 1G. My data plan has been grandfathered as well through my 3GS. However with the 4S, I've gotten warnings via SMS for the past couple of months stating the following:
"ATT Free Msg: Your data usage is among the top 5% of users. Data speeds for this bill cycle may be reduced. Visit
www.att.com/dataplans or call 8663447584."
Mind you, my daily usage habits haven't really changed from phone to phone. Between daily work emails, personal emails, surfing the web, social networking, games, maps, iMessage, etc., it seems that the iPhone has outmatched the data providers.