That's when physicists will perform the first high-energy particle collisions with the Large Hadron Collider (LHC) at CERN. Actually, the chances of anything untoward happening is extremely unlikely. This is according to physics types, who were correct in predicting that the detonation of an atomic bomb would not in fact set fire to the Earth's atmosphere, as some had theorized at the time.
The threat of a planetary-scale disaster has certainly added to the air of excitement concerning the LHC. Sure, charting new territory into our understanding of matter and the Universe is exciting in a general, abstract sort of way. But the modicum of doubt about whether or not we could really obliterate the Earth in a single experiment injects a degree of tangibility that anyone can grasp, and has a way of inspiring awe in our technological prowess, notwithstanding the unlikeliness of such an outcome.
But just how unlikely is it? I'm not a physicist, but after browsing the web a bit, I wouldn't recommend cashing in your retirement account just yet. Here's my understanding of the situation: The micro black holes that might be created by high-energy collisions in the collider would be very, very tiny. They're so tiny, they would make a proton look ginormous. If there was a black hole the size of a proton, it would weigh 2 billion metric tons, or about 10^12 kg, but the micro black holes would weigh on the order of 10^-24 kg, so the micro black holes are around 36 orders of magnitude smaller than a proton. These micro black holes are expected to be ephemeral and dissipate almost as soon as they are created by something called Hawking radiation, though there's some speculation about whether Hawking radiation actually occurs.
Even if the micro black holes hung around for some time, they would consume matter at an exceedingly slow rate: about 1 iron atom every 3 hours. At this rate, it would take about 10^46 years to consume the Earth. So these quantum-scale black holes are really not much of a threat, and could teach us something.
The only risk is if billions of the micro black holes could persist long enough to coalesce into a larger black hole, which requires some other conditions being just right (the accretion occurring exactly at the center of mass of the collision; short-range gravitational forces being stronger than expected; not getting zapped by Hawking radiation; etc.). If all these things fall into place, the accreted man-made black hole would get pulled toward the Earth's core, sucking in matter along the way and rapidly growing ever denser until all matter on Earth gets compressed into a nugget about the size of a chunk of kitty litter.
Another danger unrelated to micro black holes that has been raised concerning the LHC is the potential generation of strangelets. But this threat seems even more remote than that from micro black holes (which is pretty darn remote). Still, if things start getting "strange" in May, well, it's been good to know you!
Regardless of what happens after the LHC goes into business (new discoveries about fundamental physics or world annihilation), my only regret is that it's not happening in the U.S.
Here are some additional links about the LHC buzz: