See also: LHC alarmistsIf you do believe in theoretical physics of the last 32 years, including Hawking's semiclassical calculations and/or its confirmations by string theory, you will be almost certain that the black holes, even if they were created, will decay instantly.
If you don't believe theoretical physics, especially if you only believe some parts of it - such as the possible occurence of man-made black holes at the LHC - you may be worried. :-) Perhaps, the black holes will be produced and they will swallow the Earth.
Well, I think that even if someone believes that theoretical physics can't be trusted - and many people clearly do - there exists a less scientific argument why the accelerator won't lead to such a catastrophe: the Earth is bombed by a lot of very high-energy cosmic rays and the center-of-mass energy of the collisions is comparable to the LHC energies. So far, these collisions haven't destroyed the Earth, so it is reasonable that some additional collisions we create won't be able to do so either.
Well, yes, I am assuming that it is the center-of-mass energy that decides about the products in such a collision. In other words, I believe special relativity. If we are not allowed to use special relativity in such arguments, it will indeed become very difficult to assure anyone that the LHC is safe and that it won't destroy the Earth. ;-)