The original game may have received a mixed reaction, and all signs point to it not selling stupendously, but the word on the street is that Mirror's Edge is getting a sequel. The first-person parkour 'em up was revealed by DICE senior lighting artist Oscar Carlén on his portfolio.
All mention of the game is now gone, but before the evidence was scrubbed, it made mention of Carlén working on Battlefield 3 and Mirror's Edge 2. The fact that it was removed could confirm that it's still in development, since publishers seem quicker to deny a game being made than a game being thrown on the scrapheap. Still, EA's recent employee cull might have affected it.
It's worth noting also that Mirror's Edge was today announced for the iPhone (screens in gallery). This at least confirms that the IP is not dead, and Electronic Arts is still interested in pursuing the game as a series. If you were a fan of the game, there is still hope yet.
Until official word arrives, there's not a lot we can say. Just be aware that Mirror's Edge 2 was at least in development at some point, according to one of the developers.