Yesterday's New York Times story on Stuxnet mentions a 2008 PowerPoint presentation made by Idaho National Laboratory and Siemens at a conference in Chicago, on the vulnerabilities of the Siemens Process Control System (PCS) 7 operating system used by the Iranians. The New York Times reports that the presentation is no longer available on the Siemens website, but I have obtained a copy here:
http://graphics8.nytimes.com/packages/pdf/science/NSTB.pdf