Simply pumping against the mains pressure is not itself a difficult goal, and a pipe break wouldn't be needed. After pipe breaks in the mains are mended, generally there are public cautions not to ingest the water until it has been tested for safety (though that would be a micro-organism test, not a poison test). During the day the flow through the mains would be brisk and quickly dilute any poison; night time might be another story. Ordinary spray type pressure washers can generate thousands of PSI (although at a fairly slow flow rate) and normal water pressure is in the vicinity of 70 or so PSI. The terrorists would need to first collect a large enough supply of water, of course, to be able to send the poison back to the mains. Probably much more than a bathtub or water heater full. (Houses with a spa or a swimming pool -- which would be most likely to be found in the wealthy areas of town -- would seem to be most suspect.)
This having been said, I wonder if this whole thread should be canned. I would hate it if Free Republic turned out to be a school for terrorists.
Yeah, I guess we are treading into dangerous territory, like the discussion of improvised explosives.
Judgment call for you... is the speculation going on in this thread too dangerous (as in cluing in would-be terrorists)?
P.S. I'm glad my house is on a well. There's still the water table to be concerned about, but that has a smaller exposure to tampering than a municipal supply.