Overblown scenario ... why not something simpler and use somthing a little more untraceable preceeding the event - Molotov cocktails using gas from a 7-11?
Why not just sever every 'line' that crosses every Interstate highway 'loop' that surrounds every city?
Why not just ... just .. just ...
Propose what you wish - I say your appraisal of people who come to this country is wrong! because they DON'T WANT TO raise the kind of havoc you propose; Atta et al were a special case in my book and had single-minded sponsors with deep pockets who paid one particular single-minded person to pull off the WTC event - ONE of our BIG BIG assets in this war on terror is that - PEOPLE want to come here to GET AWAY from all that crap and simply live their lives and have an opportunity to do what they couldn't in their homne countries - live free!
In short, I think you've got a myopic view of this scenario (power system redundancy; the seeing of a control room as being *absolutely* essential; individual ops at generating stations could almost work together to overcome this) not to mention it's built upon a false premise; IMO the probability of terror with our increased world-wide efforts (as well as here in the US) is zero ...
My opinion is it's low, but nonzero. That is the nub of our disagreement.
FWIW, I hope you are right.