Schneier’s write-up of the Stuxnet worm is good:
None of this points to the Bushehr nuclear power plant in Iran, though. Best I can tell, this rumor was started by Ralph Langner, a security researcher from Germany. He labeled his theory “highly speculative,” and based it primarily on the facts that Iran had an usually high number of infections (the rumor that it had the most infections of any country seems not to be true), that the Bushehr nuclear plant is a juicy target, and that some of the other countries with high infection rates–India, Indonesia, and Pakistan–are countries where the same Russian contractor involved in Bushehr is also involved. This rumor moved into the computer press and then into the mainstream press, where it became the accepted story, without any of the origina caveats.
via Schneier on Security: Stuxnet. Make sure to check out the comments as well.