Leaving aside any attacks on the control system, the actual physical infrastructure is quite vulnerable. It operates close to breakdown on many days anyway, so it would just take the loss of some critical lines and maybe substations to cause (if lucky) widespread controlled blackouts and load shedding, or if unlucky, uncontrolled massive blackouts.
Since 9/11 they've fortified certain pieces (in the late 1990s, a 3 guys with crowbars and pistols could have broken into major grid operations centers and shut them down), but a lot of it is still fairly vulnerable.
(http://en.wikipedia.org/wiki/Fuel_protests_in_the_United_Kin...)
Some aspects of security are purely reactive (One man with a shoe bomb? All passengers now remove their shoes. One man with an underwear bomb? All passengers now go through millimetre wave scanners) and so this vulnerability has been fixed. Other protests were much less successful.
This "robot drone aircraft freakout" sounds as silly to me as an aerospace engineer as worries about malicious hackers shutting down the US power grid sound to you.