As part of the re-authorization bill passed by the congress and awaiting Bush’s signature NASA will lead a search for asteroids that could be threatening Earth. The scope of objects that NASA will be looking for are asteroids larger than 140m in diameter. Currently NASA has only been searching for asteroids larger than 1km. As part of the task, NASA will be charged with cataloging 90% of the potentially threatening asteroids in 15 years.
The largest risk associated with asteroid impacts to the Earth are those asteroids that are less than 1km because they can happen more frequently, and can trigger tsunamis. However, the asteroid that hit Siberia in 1908 and leveled thousands of kilometers of forest was only 60m.
Still, finding these 140m objects will require even more capable telescopes than are available today.
For more details see the story posted at New scientist