All this talk of attacking the reservoir or treatment plant seems like the hard way to get things done. If I were a super-evil terrorist, I’d attack the water at the place it would do the most damage (affect the most people) and be hardest to detect: in the water mains under the streets, as close to the treatment center as possible.
Maps of municipal water layouts have got to be pretty easy to come by (a walk down to the city hall; in Illinois I bet you could even call JULIE). The tools required for access would be a few worksuits and helmets, maybe some fake water company ID, and some handtools. Or you could just do it at night and get by with the tools. I don’t know much about large volume water pipe valve systems, but there’s probably an easy way to introduce contaminants into the supply without causing major leakage or disruption in service. Even if our villain had to get the bad stuff in there the hard way, it seems reasonable that he or she could research water pipes, learn to drill very small holes in one (diameter of .5 mm or less) and attach some pre-made pressure injection system.
Of course these are just five minutes’ worth of conjecture, and I don’t know much about the physics and administration surrounding municipal water systems (they’re all just quick guesses). It seems like everyone is quickly dismissing the possibility of such an attack without thinking about all of the vulnerable parts. Maybe it’s because I’ve been reading Bruce Schneier’s new book.