The way I see it is that so much vital stuff in the US is unprotected any power plant is just another target. I know only a bit about how they work, but they cannot be bombed into melt down, though I am certain some radiation could be leaked. Looking at the limits of the explosives the terrorists have at their disposal, however, I have seen no evidence they have enough power to create an explosion large enough to do that.

If we want to talk about scary terrorist targets that are unprotected, let's talk water supply. There is simply no physical way to protect it, and a coordinated attack with the right stuff in the right places could be more crippling than anything else I can imagine.

I can't say about the wind plants, but the coal facilities certainly have had their security beefed up post 9/11. They may lack the gun towers, automatic weapons, and body armour, but they do have armed gaurds patroling the grounds, manning the various check points, and doing thorough vehicle inspections - interior, truck space, engin compartment and under the car.