First the good news…

All very large asteroids at risk of hitting Earth should be identified fairly soon:

"In a few more years, we'll be able to say that there's nothing out there to cause a global catastrophe,” said David Morrison, director of the NASA Lunar Science Institute and senior scientist for Astrobiology at NASA's Ames Research Center.

Then the bad news:

“But, there'll be a million that will be big enough to wipe out an entire city. It'll take a long time, if ever, to find them and figure out their orbits. The bottom line is, we could be hit by one of those small ones at any time, with no warning at all. Right now, I can say almost nothing about the probability of one of those small objects hitting us, because we simply haven't found all of them."

The article also notes that some scientists still think a nuclear bomb would be the best way of dealing with a large, threatening, asteroid.