For you old-timers who stuck with AT&T's unlimited data plan since your first iPhone back in 2007, AT&T thanks you and now wants more money.
The 1st paragraph of the email sent to users who had their unlimited Data plans grandfathered along with them over the year reads:
Upcoming Changes To Your Data Plan
..we'd like to thank you for your business and share an upcoming change. Beginning February 2016 we'll be increasing the price of your unlimited data plan by $5 per month our first increase in 7 years.