From the Guardian:
"Maj. Gen. Thomas Cutler, who leads the Michigan National Guard, said he anticipated a call for police units and started preparing them, but couldn't go until states in the hurricane zone asked them to come. 'We could have had people on the road Tuesday,' Cutler said. 'We have to wait and respond to their need.'"
"Bush had the legal authority to order the National Guard to the disaster area himself, as he did after the Sept. 11, 2001 attacks . But the troops four years ago were deployed for national security protection, and presidents of both parties traditionally defer to governors to deploy their own National Guardsmen and request help from other states when it comes to natural disasters. "
That last quote is a little misleading. The President can't just "order the National Guard to the disaster area", and he didn't in the case of New York.
In order for the Guard to deploy under the Presidents orders, they have to be mobilized under USC Title 10 or Title 32 as this nifty little graphic from the National Guard Bureau slide show "The National Guard 101" clearly shows:
This is so much the same arguement that I have made before about many people not understanding that the United States is a REPUBLIC and that the states are independent entities.