After studying the Stuxnet worm for months, researchers have determined that it is designed to invade industrial control systems used by factories and power plants, check the configuration of the system to see if it's the one it's intended to sabotage, and then control the site's machines to cause mayhem.
The worm was not spread through the Internet, but apparently by the USB drives of a major international contractor who had several clients, any of whom could be the target. It doesn't need any human interaction to operate, beyond merely plugging in the USB drive, which immediately loads the worm into the system. The sophistication and abilities of the worm, which were thought to be impossible, indicate it was created by a team employed by a nation, or someone with the resources of a nation, which could support the knowledge base and equipment needed to create it. Assuming the experts aren't overestimating the sophistication needed and find it was actually a kid using a school computer.
The activity of the worm indicates that it's looking for an Iranian company, possibly the Bushehr nuclear power plant that Iran is implementing despite opposition from other nations, including the U.S. The consensus is that the worm has already found its target and is waiting for the right conditions to activate, like the startup of the Bushehr plant, or it's already done its damage and either no one has noticed or it's being kept a secret.
Whatever the impact the worm has on its target, the impact on the computer industry and world security is chilling. It's proof that malware weapons of mass destruction are not only possible, they exist, and have been used. The genie's escaped, and now the world's wrong hands will be buying and selling and using them.
Christian Science Monitor article.