For those with Radeon cards, install the catalyst drivers version 3.9. Not 4.1, or 4.2, only 3.9! Anti aliasing only works with the 3.9 drivers. I used to have that problem, and I've seen other have that problem, too, and anti aliasing was always the problem.
roshjast, you say you had AA turned off in the game settings, but did you turn it off on the desktop settings window? Turn it from there, too, if you don't want to install the 3.9 drivers.