The fear was that it'd create a black hole when turned on, but that didn't happen so we're good... for now. As far as I know the experiment hasn't been done yet, I believe it takes 1-2 weeks to get the atom going the speed of light before they collide it to try and recreate the big bang theory. It's really a fascinating breakthrough, whether it works or not.
Actually, if the experiment works, it'd create mini black holes that could still prove disastrous. It'd be funny actually, mostly in the sense that doomsday will start in a neutral country =]