Black Holes tend to have a chain-reaction nature about them...
Only if the event horizon exists long enough to be able to start accumulating mass. Of course the high velocity any potential black hole would gain from the momentum of the particles that formed the event horizon would mean that time dilatation would slightly lengthen the apparent lifetime of such a black hole, but even so any black hole with mass equivalent of energies released in LHC experiments would dissipate rather fast, or so the theory says.
Mind you, most of the energy in these experiments is kinetic energy. It causes interesting stuff to happen but it's still a relative form of energy and according to my understanding it wouldn't just "morph" into rest mass just like that. Any black hole would only have as much mass as the particles had; rest of the energy would be either released as radiation or become the black hole's (or other particles') kinetic energy. This velocity would be well over the escape velocity from solar system... or the galaxy even. Couple this with the fact that the diameter of the event horizon that would be borne would be really really tiny, so tiny in fact that the probability of the hole actually hitting a particle would be minuscule - even if it were to travel through the Earth.
So sayeth the wise Alaundo.
Repeating the best argument on why LHC will not cause end of world in any shape or form - it hasn't happened yet, and LHC is not going to be doing anything that wouldn't be happening all the time naturally anyways except making high energy particle collisions repeatable and thus a lot easier to research. Very high energy particles hit
you all the time. Looking at an open, unprotected bubble chamber it keeps flashing all the time from all kinds of particles hitting it - you might want to check if there's one near you at an observatory or science center or whatever, it's kinda like traceroute...
Anyhows - LHC will instigate proton-proton-collisions of 7+7 TeV, which means 14 TeV total collision energy, or 14*10^12 electronvolts.
Cosmic rays can have energies of over 10^20 electronvolts. Or, if you will, 10 million TeV. The highest observed occurrence of a cosmic ray was a particle with energy of 50 joules which is about the same as the kinetic energy of a tennis ball flying at 42 m/s, which is pretty fast. If it was a proton, it was traveling so close to light speed that it would travel one light year minus 46 nanometers in a year...

Since these particles haven't caused the destruction of Earth, It's very safe to say that LHC will not do it either.
Here's something of interest by the way... a simulation about what happens when 1 TeV proton hits the atmosphere:
http://astro.uchicago.edu/cosmus/projects/aires/