The CRS Take on the Military as Disaster Central:

It seems that everyone from President Bush on down is talking about realigning the military’s role in disaster relief operations in the United States—after the devastating effect of Hurricane Katrina on the Gulf Coast. However, military operations on US soil are touchy subjects and are governed by several laws, according to a recent Congressional Research Service report on the topic. The Stafford Act, for example, authorizes the use of the military for disaster relief at the request of a state governor—but does not permit the use of military assets to perform law enforcement. However, the President can sidestep the Posse Comitatus Act by invoking the Insurrection Act to use federal troops to aid in the execution of law.