The end of the World

I think there are currently two ways for mad scientists to destroy the world.

1. The new accelerator at CERN has a sort of chance of creating quantum black holes, depending apparently on how tightly the insensible dimensions are rolled up. Good looking theory says that they should evaporate in a tiny gamma burst, but no-one can be sure how relativity works at that scale. If they don't evaporate, they will fall to the earth's core and then consume the whole planet. The first few atoms might take a while, but after that, they'll be unstoppable.
2. Craig Venter's team expect to be loading their carefully written DNA into a cell this year. If they cock up and build a replicator, there's no easy telling what it'll manage.

In the past, the best bet was fusion bomb tests: could they ignite light elements in the planet's crust? Well, no as it happened, and I'll guess we'll be alright with these two, too.