Well, right now? Not a whole lot. Duck, I guess. For all of you who remember the Movie Armageddon, believe it or not, there was little if any realistic scenarios for dealing with the asteroid of doom. Here is an article adressing these issues- To Nuke or To Nudge
My problem is that currently we have nothing in place to address this problem. We do have many programs addressing the tracking of these bodies, NEAT for instance, but none addressing the "ok, we found one that's going to hit- what do we do now?" part.....
There is a good debate here concerning the feasability of actually even worrying about this issue, and what can we do about it, as well as more recently this conference here where the issue seems to be gaining some ground within the Space community.
All I'm saying (and my friends reading this right now are rolling there eyes) is that we should move this issue up the ladder in terms of priorities within the Space Program, because most of the other programs will not matter much once an asteroid on course for the earth is detected.