Above: Train them now! An oversized head? Make sure it's dead!
The regular "Examiner" column at Slate answers the question: "Do we have an alien-contact plan in place?"
The answer: Not for any situation where we'd really need it. There is an existing protocol, proposed by SETI, for reacting to the discovery of a signal from the far reaches of space.
The protocol, adopted in 1989, is that if someone detects a radio signal seemingly indicating that we're not alone, he should get in touch with SETI researchers, who will help him verify whether the signal is really and truly evidence of extraterrestrial intelligence. At that point, he should notify the International Astronomical Union as well as the United Nations and relevant research organizations. On the finders-keepers principle, the discoverer would get to make the first public announcement, but data should be made available to the international scientific community. (Source coordinates, however, would be kept secret, to avoid a situation in which anyone with a radio telescope could start up a conversation.)
And that's all Kool and the gang; but what we all really want to know is if there's a plan for dealing with aliens who show up all determined to blow up our major landmarks, kill our leading citizens, and run off with our women folk. Aside from "contact Will Smith," there is no plan.
In the farfetched Hollywood scenario wherein we detect an alien spaceship or aliens send us a Greetings, Earthlings!-type message—all bets are off. (If the Pentagon or some such has a plan for how to deal with contact, it's classified.) Naturally the response would hinge on the nature of the contact: peaceful or violent, needy (give us fossil fuels) or helpful (cold fusion). Many scientists, including Stephen Hawking, believe that contact with intelligent aliens would end badly for us—we'd be the Native Americans to the alien Europeans. "I imagine they might exist in massive ships," Hawking said recently, "having used up all the resources from their home planet. Such advanced aliens would perhaps become nomads, looking to conquer and colonize whatever planets they can reach."
Lacking official protocol, those worried about first contact can turn to the very unofficial Introduction to Planetary Defense: A Study of Modern Warfare Applied to Extra-Terrestrial Invasion. Like Hawking, the authors believe humans would play the part of Native Americans circa 1492. They also think that, in light of the sluggish global response to natural disasters, there's little indication that we could react effectively to invasion. Since we'll probably be technologically outmatched, the best defense strategy would be guerilla warfare.
So we graffiti alien installations with "Martians Go Home" and start making Molotov cocktails. Apparently plan A is more Red Dawn than ID4.