MANPADs are man-portable air defense systems (SA-7s, SA-14s, Stinger missiles), and pose one of the greatest terrorist threats to the United States. Secretary of State Colin Powell has warned that "no threat is more serious to aviation" than MANPADs. They actually have legitimate military use, but are particularly worrisome when they fall into the hands of terrorists.
The US government has been actively exploring ways to counter the threat posed to aircraft by MANPADs. Now comes a RAND study confirming what I had guessed already: it would be way too costly to outfit the US commercial fleet with appropriate anti-missile technology, and likely would be overcome by terrorist modifications anyway.
One of the best ways to counter the MANPADs threat is to teach pilots how to evade them:
"Successful evasion is a low-cost, near-term solution to the threat. A trained pilot can be very effective in evading missiles. Thus, a relatively low-cost and efficient near-term response to the missile threat is to provide pilots and air controllers with training regarding evasion procedures."
Anyone who has flown in or out of Baghdad can tell you what a pleasant experience being on a plane taking evasive maneuvers from MANPADs can be. Basically take the most exciting, hair-rasing roller coaster that you've ever ridden, and throw in longer and bigger dips, bobs, and weaves and you'll get the idea.
One final note: the National Intelligence Reform Act of 2004 contains a provision that dramatically increases the penalty for possession, use, import or acquisition of MANPADs and their components. Believe it or not, prior to this legislation the US code treated possesion of a MANPAD as a violation of the firearms laws, with a shockingly low penalty. There is no legitimate reason for a private citizen to possess a MANPAD, and now any individual who does so will be punished appropriately.