Don't know about the power grid, but I think at least in Europe it seems to be networked among several countries. If one country does not produce enough power, it can import power from other countries. Unless there is a single main line between the countries, it seems difficult to destroy.
On the contrary, it's a huge problem. Sooner or later there will be the surge they can't deal with and then we could easily get the same cascade issues that destabilized the North American grid during the outage. Island mode - the disconnection of all external connections - is the failsafe of last resort but by that time there will be large areas of Europe in the dark ages.
The grid has become so interconnected with such highly fluctuating loads that it's a miracle we haven't seen a serious outage yet.
Basically Germany uses Polish, Czech and Hungarian grids as a load leveler and as a way to transport power inside the union to countries they have deals with. But exactly those grids are amongst the oldest and weakest that we've got.
This is not a simple problem and the potential for error is much larger than you'd expect given the number of people that rely on these systems. So far so good. I personally believe that Germany should pay for the conditioning equipment to deal with the varying load of their windplants, the load patterns are a direct consequence of using wind energy and it should not fall to Germany's neighbours to fund a chunk of infrastructure that solely exists to deal with the wind surges.
In the United States every windplant has a conditioner at the uplink to make sure that the relatively fragile long distance network does not suffer. That's one single economic entity so there is no externalization possible.
Technically it is trivial to fix the overload by disconnecting part of the generators (at their entry-point, not an "island mode") so that the grid will survive - it can be even done automatically in a fraction of a second, although the wind generators themselves will suffer damage.
The problem is economical/political - who will have the authority to say that we'll disconnect generator X and won't pay for it's electricity since noone needs it at that specific time.
So the terrorist plot would be to overload the power grid. Since they come from oil rich countries, they'd probably have the spare energy to do that :-) Place some oil powered generators at strategic locations and blam - darkness...
But in seriousness, I find the issue interesting, but it seems solvable.
