There is an interesting article on a CNET blog about the Worm Simulator that Symantec have developed. Apparently it's an update to their old VBSim application that was released in 1997.
The tool, available for free download, displays an image of a rotating globe and one of a typical corporate network. As a worm starts to slither around the Net, the globe will turn red as systems get hit in different geographies. The network will also go red as the worm wriggles its way in.
Once they get the facility to "use" your own network you will be able to see how and where you might be penetrated by the worms. Do Symantec already offer some sort of system where they will monitor your own network for intrusions?