The folks at NASA now have a twitter account for announcing asteroid tracks. Here is the link:
Better yet, why not get the new asteroiod widget for your Mac? Not only do you know what ateroids are out there, but you also get a handy guide to how big they are. For example, August 8th 2009, there will be an asteroid passing us the size of a four story building! That's putting the gravity of the situation right on the old Apple!