Phil Plaint debunks myths related to astronomy on the blog Bad Astronomy. He did a speech at TEDx Boulder in September and outlined two ways in which we can defend Earth from asteroids: we hit them with space probes or simply send a space probe heavy enough that it will drag the asteroid from its course.
How can the second method work? Well, gravity can do magic if you know to use it. The probe would be linked with the asteroid by the gravity that would keeps them both together. If the probe moves slowly from the path the asteroid has if can move it. Did you hear about ion thrusters? Well, we already have them and they will be in use by the 2020ties when we might encounter Apophis on his way.
So, there is hope for everyone. Should we use ka-booms or simply use the gentle touch of gravity? Well, blow that thing up before you even see it good. The way to go 😀