That may be changing. The United Nations Scientific and Technical Subcommittee has a special working group on NEOs advised by a special coalition of experts (called Action Team 14), ranging from space agencies to advocacy groups. Together, they are working to formalize an U.N. framework for coordinating an international response to potentially dangerous NEOs. Their recommendations, which will be presented in February, are expected to include proposals to establish two institutions. The first is an international asteroid warning network to coordinate the search for NEOs, determine which ones are threats, and what their characteristics are (what they're made of, how fast they're traveling, and other factors relevant to trying to deflect them). The second group is a "space mission planning advisory group" -- comprised of engineers, astronauts (and cosmonauts, and maybe taikonauts), and other representatives of various countries' space agencies -- to plan potential responses to threats. It's a good start, but some experts feel it's moving too slowly (surprising no one familiar with the United Nations).
For now, it would probably fall to the few countries with advanced space-launch technology and powerful ballistic kinetic weapons -- the United States and Russia, most prominently -- to try to divert an incoming NEO. In testimony presented to Congress in 2007, NASA proposed two broad categories for methods to deflect an NEO: impulsive technology, which would use explosives to alter the object's course; and "slow push" technologies, which would use everything from directed solar energy to gravity wells to gradually shift the object away from impact with Earth. The effectiveness of these methods would depend on the size of the NEO and the time available -- and as NASA cautioned in its report, much of the technology is years from being ready for launch.
Experts' best estimates for the time necessary to successfully divert an NEO from hitting the planet is, at bare minimum, two to three years, although five years is probably more realistic. Given that it's impossible to get a three-mile hunk of rock hurtling through space to make a 90 degree right turn, small deflections far from Earth are required. And it takes time to prepare a spacecraft and fly it out to meet an asteroid deep in the solar system. With many of the slow push technologies still years away from being practicable, it would require brute force -- either crashing a spacecraft into the object or altering its course with nuclear explosions (delivered by unmanned rockets, not oil crews). Here there be legal trouble, though: using nukes would technically violate the U.N. Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space.
At this point, say experts, we're not quite ready to muster an international effort to save the planet from space rock disaster. But at least governments are getting more serious about the problem and taking the responsibility for detection from armchair astronomers. And should we all make it past December 21, 2012, don't start celebrating just yet. 4179 Toutatis, the three-mile wide asteroid that passed by yesterday, will be zipping by again in four years.