[link|http://news.bbc.co.uk/2/hi/science/nature/2147879.stm|Here's] a story about a fairly large asteroid on a collision course with Earth.
Don't soil your pants just yet, though; it's 15 years away and they'll certainly need more time to compute and recompute its orbit.
(On the other hand, wouldn't it be ironic if they decided it wasn't a threat, then some astronomical perturbation put it back on a collision course?)