The Posse Comitatus Act precludes the use of the military as a police force within the states under normal conditions. Due to the Insurrection Act the President also is limited in how the National Guard can be used. Because of the response of the State of Louisiana during hurricane Katrina, Posse Comitatus was amended in 2006 to allow the use of the military during natural and man-made disasters (Thank you Janet Napolitano!). In 2008 Posse Comitatus was re-amended and the new language was stripped out.
http://en.wikipedia.org/wiki/Posse_Comitatus_Act