There is also the problem of Black Start if a wide area grid outage occurs.

A terrorist network attack of the electrical grid targeting the transmission lines from black start generator sets could be absolutely devastating to a countries economy enough to cause chaos from many a megacity (which itself could cause other Megacities to fail esp if the initial targeted Megacity was a finance centre) , the clock may easily get past that 3-4 days grace. 7+ days could be extremely challenging.

Knocking over the grid initially would be relativity simple especially during the high heat of the summer.

http://www.youtube.com/watch?v=BGJHwzsPdpY