Several opponents to the project fear the collision will form a mini-black hole (which could swallow the Earth whole, in theory). I tend to side with the guys who have designed the experiment, but, as I asserted before, I know jack shit about physics.
I do know, however, that many of the physicists who designed the first A-bomb feared it would trigger an endless chain reaction that would burn up the planet. I think nuclear weapons are an awful scourge, but we are still here. My guess is that we will all wake up Thursday morning in good form.