To tell the honest truth IMHO, whatever we do, our only real hope is to plan ahead, this discussion should be taking place way before now and action should already be in place. I know others have discussed this but here we are with nothing to protect us... action is needed.
I think the current worry is over Apophis, some 300 meters across? Imagine the cluster fuck if it was found that this asteroid was going to hit instead of passing us by in a few years, could we do anything even with such a long warning time?
The time to have started this was 20 years ago, anything we do will require years of lead up time, prototyping, testing, not to mention getting international laws bypassed.
This should be a major worry for everyone.
A side note, if we could, wouldn't be better to direct an asteroid into the Moon or another body, just to make sure it doesn't come back around in a thousand years or so, or would this just open another can of worms?