After seeing the Taliban, and any other government helping Bin Laden, forcibly removed from power, I think any remaining rogue states out there would have very serious second thoughts about ever threatening the US in such an overt and obvious manner of firing off a missile.

Plus it's pretty clear now that the real and immediate terrorist threat is much more down to earth, and that all our resources should be concentrated on identifying and stopping that shadowy lunatic who's going to sneak a small nuclear device into our country, not deliver it from above.
Without a state sponsor participating, a group like Bin Laden's doesn't have the capacity to fire missiles at us, but they sure could smuggle a nuke in.

I admit that afer Sep. 11, I've had a change of heart on this issue - the missile defense system may still be a good idea someday, but it should be put on hold for now.