For those who don't know the Stuxnet worm was the first computer worm to specifically target SCADA (Supervisory Control and Data Acquisition) systems and also the first known worm to install a rootkit on a PLC (Programmable Logic Controller). It is largely regarded as the most sophisticated piece of computer malware to date. Experts worry that this source code could be studied by attackers to create similar sophisticated attacks.
For a more in-depth look at what Stuxnet is I suggest reading over the Wikipedia entry or watch this youtube video by Semantec demonstrating what Stuxnet does to a PLC.
The decompiled source code can be found here, and for more news and updates on the source code follow @stuxnetsource.
I link to the source because I believe knowledge is for everyone and the more people that can study these types of malware the better off we all are because the more wide spread the knowledge the less powerful it becomes.