The military services were used for civilian law enforcement in 1992 when President Bush issued an Executive Order for the US Marines, Army, and the federalized California National Guard to restore order following the verdict of the Rodney King trial.
So much for the "President doesn't have the authority" rap.
Additional non-combat domestic duties were added. In 1992, the US Army and Air Force assumed a domestic disaster relief mission when they responded to assist after Hurricane Andrew in south Florida and after Hurricane Iniki in Hawaii a month later.
A precedent for hurricanes.
http://powerlineblog.com/archives/011542.php
Another view
Below is a link to an excellent summary of the National Guard and reserves. The paragraph below is from this. This writeup was published in 2002.
Maybe some Freepers in the National Guard will come on board and explain Federalization of the Guard and how it can happen.
http://www.stormingmedia.us/63/6350/A635024.html
"In peacetime, however, National Guard units belong to states, and state governors are the commanders in chief. Unless federalized, Guard members are not subject to the Uniform Code of Military Justice, and Guard units fall outside of the formal Department of Defense (DoD) command structure. Under the law, the National Guard is composed of individual, but nationally funded and regulated state militias that can be federalized and used as a reserve force."