Symantec researcher: Stuxnet can take control of programmable logic controllers (PLCs) and disable machinery connected to them. Liam O Murchu described a sophisticated and highly targeted virus and demonstrated a proof of concept exploit that showed how the virus could cause machines using infected PLCs to run out of control. Without understanding what types of machinery the targeted logic controllers were connected to, it is impossible to know what harm the worm caused on infected industrial control systems if any.
Source: https://threatpost.com/stuxnet-analysis-supports-iran-israel-connections-093010/74538/