In an updated version of its analysis of the Stuxnet worm that caused major problems for Iran's nuclear plant earlier this year, Symantec finds that it's likely the worm was designed to take down critical infrastructure in nuclear plants. It's designed to subtly sabotage the controls of a nuclear plant by speeding up and slowing down machinery in the facility. Not enough to break things, just enough to screw everything up pas the point of functionality and not get caught in the process. The worm went after a particular kind of control system - it was made to intercept commands from that control system for a particular function. It's that function that has been mysterious so far, but researchers have found the worm is looking for some very detailed specifications.
From Wired:

It inventories a plant's network and only springs to life if the plant has at least 33 frequency converter drives made by Fararo Paya in Teheran, Iran, or by the Finland-based Vacon.

Even more specifically, Stuxnet targets only frequency drives from these two companies that are running at high speeds - between 807Hz and 1210Hz. Such high speeds are used only for select applications. Symantec is careful not to say definitively that Stuxnet was targeting a nuclear facility, but notes that "frequency converter drives that output over 600Hz are regulated for export in the United States by the Nuclear Regulatory Commission as they can be used for uranium enrichment."

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