The sky isn't falling on our heads, but it's getting close. Toutatis - a 5km long asteroid - missed the earth by a mere 1.6 million km today. It's orbit is well known, and this is the closest it will come for at least 500 years, but it is a reminder that there are plenty of similar-sized rocks out there whose orbit is not known, and which could pose a danger even in the near future. While a rock the size of Toutatis doesn't pose a threat to the existence of humanity (we and our technological infrastructure are too widely dispersed to be taken out in one hit), it could make things very difficult for quite some time.
Lembit Opik takes the opportunity to call for an early-warnng system to track asteroids and warn us of any danger.