[P-Marlowe]

Once the water is in the water system, the water pressure alone will keep out any attempt to insert contaminants. They could put it into water towers, but it is probably still going to be diluted to the point of ineffectiveness.

What they might accomplish is to put a little panic into the people. A lot of people might freak out if the water supply was contaminated, but how many Italians actually drink WATER? Most Europoeans don't bathe too often either. So what would be gained by poisoning the water supply in Europe?

[piasa]

True... it would be more effective to poison the vino or food and probably easier, given the lack of security in most food processing plants.

Panic and publicity, of course, is the usual desire of terrorists. If it hurts politically or economically, it is useful to them whether or not they kill large numbers of people.

BTW, cyanide can also make a pretty nasty gas. That was the lamebrain idea the first WTC bombers had.

[HiTech RedNeck]

Ever heard of pumps? Acrobatics to get into water towers would not be necessary. And as a bonus, the water company might end up sending them a refund as the meter runs backward. (OK I was kidding about that last part.)

[e_engineer]

Here is a thread that discusses how they would probably use the water system to spread the poison:

Backflow Attacks