Engineering and Computer Simulations (ECS) creates virtual worlds based training and simlation aids such as US Nexus. Demonstration videos for projects such as the US Joint State Response Training System (JSRTS) are available online.
Videos: http://www.ecsorl.com/gallery/video/EmNexusVideo.html (e.g. check out JSRTS Video on left hand menu)
US government uses and lead: http://www.fcw.com/Articles/2009/05/04/Feature-virtual-learning.aspx
Presentation on US government uses: http://www.simulationinformation.com/Gametech09/presentations/Panel_WorldsofPromise.pdf [PDF File]