http://en.wikipedia.org/wiki/Stuxnet
http://www.foxnews.com/scitech/2010/12/09/despite-iranian-claims-stuxnet-worm-causing-nuclear-havoc/
This thing is fascinating. The stuff of great novels.
Iran's nuclear enrichment program is still down. The worm primarily targets PLUs on Iranian and Finnish (primarily sold to Iran) SCADA device controllers. Uses multiple windows vulnerabilities that Microsoft had no knowledge of.
With each patch to the system, the worm exploits another unknown weakness. They are calling it the equivalent of a cyber missile, and a prototype for a new arms race.