I've always understood Weapons of Mass Destruction to be things like nuclear weapons, toxic gasses, and biological weapons, devices and substances intended to kill indiscriminantly and in massive numbers.

Bombs made in pressure cookers, such as the ones used in Boston would definitely fit the "Improvised Explosive Device" catagory, intended to kill or injure as many people as possible, but incapable of killing hundreds of thousands like a nuclear weapon, nerve gas, which can drift on the prevailing winds, or a virus that can infect millions.

An Improvised Explosive Device is obviously a vicious enough weapon without having to resort to some overenthusiastic newsman's hyperbola.

Don Firth