Anyone that knows anything about the utility systems (power, water, gas, sewer) knows they are run by the most insecure systems out there. There have been reports of SCADA vulnerabilities for years (GOOGLE it yourself) and many concerns expressed (http://www.darkreading.com/blog/archives/2009/04/scada_security.html).
Once someone can get out of a compromised system and onto a utilities internal SCADA network they have total control. The best place to practice is on a relatively insecure 2nd or 3rd world network where there are few legal consequences to hacking. Once the skills are honed up and scripts can be written to control large numbers of SCADA systems at once the real fun can begin. This is one of the major objectives of terrorists (http://www.pbs.org/wgbh/pages/frontline/shows/cyberwar/vulnerable/scada.html).