I can't take grey goo seriously as a threat. We deal with brown goo - that is, biological reproducers - constantly. And our biggest problem with fighting off brown goo isn't that we can't destroy it, but that it shares so much in common with us that we can't destroy it without destroying ourselves as well. We have to use highly specialized weaponry to fight brown goo.

Gray goo, on the other hand, is going to have sufficient differences from us that we can use less discriminating weaponry - EMPs, for example, which are largely harmless to us.