Where's Sam Jackson when you need him? Polonium on a Plane could be a great sequel! There is a storm in a teacup at the moment about planes being grounded due to Polonium being found. before I address that, it's probably best to get a bee out of my bonnet.
Radiation isn't inherently bad, if you get cancer treatment you are likely to be exposed to a gamma emitter, alpha emitters (often Americium) are used in smoke detectors (read the small print, there is a limit to the number of smoke detectors you can put in domestic waste for that reason) - and beta sources can be used to help check fill levels of containers, or the thickness of sheet metal as it's rolled flat. Gamma sources can also be used to sterilise medical equipment in remote areas.
The big issue here is the fact that someone was murdered. If a state was involved, then there are diplomatic repurcussions, if a state were not involved then we have the problem is individuals gaining access to large quantities of Polonium (not an easy substance to get hold of).
Speaking as someone who knows a not insubstantial amount about physics, unless the Polonium on the Plane is in extremely high doses then it's not worth worrying about as a member of the public... alphas get blocked by a few cm of air and so unless you're in direct contact with the emitter, it's not a problem. Even then, skin is pretty good at blocking alphas! Even sitting next to the person carrying the Polonium would have been fine.
Ingesting an alpha emitter is much more of a problem, as the thing doing the blocking is your gut itself. So, unless you're licking the plane (or getting Polonium on your hands and subsequently ingesting it) then don't sweat it. The one potential issue is if the Polonium was split on the plane, this could then be picked up.
Ignoring background radiation, the fact that everything emits some amount of radiation; Merely being near a radioactive emitter doesn't in turn make something radioactive (if it did, everything would become radioactive).
As for why the Polonium wasn't picked up in screening, it's virtually impossible to detect an alpha emitter, the alphas can be stopped by a few cm of air, or a sheet of paper. Therefore ANY sort of container would render the Polonium undetectable.
When I was growing up, we had Johnny Ball on TV to explain science issues to the public. Where is the modern day equivalent? (No, I don't want someone who fits the 'boffin' stereotype, I want someone knowledgeable, enthusiastic, able to talk to children (without talking down to them) and adults alike - someone who assumes intelligence in their listener. Johnny Ball is still around... he'd fit the bill nicely!