To help you guys with any confusion that there is...
Generally speaking, AT&T allows it's customers to upgrade every 18 months.
My 2 years deal was originally signed on December 29, 2009. According to my AT&T Account page, I am eligible for an upgrade on May 29, 2011. 18 months later.
Just call and ask when you are eligible for an "equipment upgrade at the new customer price."