I'm not worried about some internet group getting into the systems remotely. A Stuxnet-type attack is definitely possible, but smart protocols (no unauthorized electronics, thumb drives, etc. on site) will make this very hard. Someone will eventually goof up, but even then there are so many overrides that executing a safe shutdown is possible even if the control systems are hacked.
I think a physical on-site attack is far more probable and worrisome (terrorists with guns taking control of a plant). There is a lot of security around U.S. plants these days, but a whole lot of complacency has built up since 9/11 and a few thousand days of nothing happening takes a toll. 20 well-armed jihadis ready to give their all for Allah and their 72 virgins could probably get into a plant. What they could do from there, who knows. Simply getting into a containment and draining a reactor pool would be pretty bad if there was a significant amount of fuel stored (which is the case in a lot of old plants) but containable. They would have to figure out how to shut down and/or disable a lot of safety controls to do anything serious. The plant itself would fight them pretty hard. If they got physical access to the containment and tried to blow up stuff, could be bad but likely containable. PWRs have systems to cope with large break loss-of-coolant accidents, which is pretty much a massive steam explosion and loss of core cooling, as bad as things get.
I honestly don't think terrorists could do anything that would cause anything worse than contained damage and contamination, nothing that would harm the neighbors. However, given the FUD already circulating about nuclear power (yeah, I'm looking at you Mr. Burne) I think it would be enough for them to just take the plant and then sit around drinking coffee. Even if they did no damage at all to the plant, got mowed down by the good guys in 10 minutes, the simple act would have the world shitting bricks. And that is what terrorism is all about, stirring up unaccountable fear.