Time magazine carries a report on arrests made in a plot to detonate a dirty radioactive bomb in London.

Reports on the British investigation, now circulating among U.S. law-enforcement agencies, assert that the group was trying to construct a crude radiological dirty bomb. The arrests (which followed a yearlong surveillance operation, code-named Operation Spangle) turned up a cache of household smoke detectors, which the British suspect the group wanted to cannibalize for their minute quantities of americium-241, a man-made radioactive chemical.
Officials tell TIME it’s extremely unlikely that enough americium could be harvested from smoke detectors to create a device potent enough to inflict radiation sickness, let alone kill people. But others argue that spewing even a small amount of radioactive material into a crowded stadium or subway station could trigger sensitive radiation sensors, incite panic and cause long-lasting contamination.

Perhaps I could chime in as your self-appointed weird metals correspondent? (Weird metals are, after all, what I do for a living).
Americum oxide is indeed used in smoke detectors, and yes it is radioactive. However, annual global usage is around 5 kg a a year. About 11 pounds spread over 6 billion people. So you’re going to need an awfully large percentage of all smoke detectors in the world to get a useful amount.
Americum is also only very lightly radioactive. It’s an alpha emitter, the particles don’t even go through paper, let alone skin. In fact, that’s why it’s used. The radiation from decay can move through clear air and set off the detector, but cannot pass through smoky air. It is the abscence of radiation hitting the detector which sets off the alarm.
It does not dissolve in the body, rather passing straight through. Even if it got into your lungs the minimal power of the alpha particles would not do you any harm. You’d get worse lung damage (and do) by having a BBQ once a month.
So, the physical damage done by an Americum containing bomb would be nothing, and the possibility of getting enough of it to actually do anything useful is minimal.
The lasting stigma would be zero: releasing americum oxide in any quantity likely to be available would not change the background radiation level. I’d want to check up on it before making a declaratory statement but at a guess americum oxide is less radioactive than the granite sometimes used to make statues (granite is relatively high in uranium and as such releases radioactive radon into the atmosphere as a result of the decay products). Relasing the few grammes that someone might be able to scrape together from fire detectors would not be noticeable a few hours later and probably undetectable in 24, except in an enclosed space.
All that said, the terror aspects are rather different. The general public’s inability to understand radiation and risk is such that someone standing in the Tube (subway) with a Geiger counter set to too high a sensitivity could set off a mass panic and a number of deaths by crushing.