Now, Didier Massonnet and Benoît Meyssignac of France's National Centre for Space Studies have come up with a new projectile to fire at the asteroid in such a "kill". They advocate capturing a small, 40 meter asteroid and "parking" it a stable Lagrange point 1.5 million kilometres from Earth, where the gravity of the Earth and the Sun balance.
http://www.newscientistspace.com/article.ns?id=dn9063&feedId=online-news_rss20
[linked from Junkscience]

Sounds good to me, but the small rock seems too big. Assuming a density of 1 & spherical: V = 4/3 × pi ×r3, so: it's mass is 35,510 tonnes. Now, it might be good size to mine. Hmm, if it had 10% it's of mass as water that could be extracted, that's 3500 tonnes or about 7.7 million pound of water, if this water was worth $1000 per lb it's 7.7 billion worth of water, if your costs were say less than 3 billion to mine it, could be economically possible to do this. Of course it could have dual use- mine water and make rocket fuel, use rocket fuel to move rock so it intersects path of a Earth impactor and if not needed sell water and or rocket fuel. Actually could have additional purpose- provide water and rocket fuel for Manned Mars and be assembly point for Manned Mars mission.

More I think about probably better to go with the bigger 40 meter rock, might be big enough to detect, big enough to mine, and not so big that it's a danger to Earth, and you can propel 100-1000 ton chunks of it at incoming impactor.