The New York Times cites unnamed sources as confirming that the Stuxnet worm was part of an American-Israeli project to sabotage the Iranian program.
Due to its complexity, researchers believed that Stuxnet was created by a state-sponsored team of programmers, and Iran has suspected that Western countries were behind the effort. The Times article seems to confirm that. It quotes an American expert on nuclear intelligence as saying:
The reason the worm has been effective is that the Israelis tried it out.
According to Computerworld, the Idaho National Laboratory helped experts at Siemens identify vulnerabilities in its SCADA systems. Meanwhile, Israel tested Stuxnet on an unknown number of gas centrifuges that were identical to the ones used by Iran.