In my earlier post "Posse Comitatus and the Use of Troops" I maintained that federal armed forces assistance is available to the governments of the states in an emergancy if application is made for such assistance under the "Stafford Act." This act creates an exception to the "Posse Comitatus" act on the basis of a state’s desire to be helped.
Larry Johnson has researched the "paper trail" on what happened and has written on it. There appears to be some disagreement among the White House, FEMA, and Governor Blanco about this.
It looks to me that Blanco made her request before the emergency.
Go take a look.