The Mirror's Edge follow-up is real and it's coming from DICE and EA "when it's ready," according to the trailer shown at EA's E3 press conference today. It runs on Frostbite 3.
EA launched Mirror's Edge in 2008 on the Xbox 360 and PS3, and later on PC. Since that release, fans have clamored for a sequel and found hope in scattered rumors over the years. The most recent, most concrete rumor was an EA help page for Mirror's Edge 2 that went live briefly a few weeks ago.