Polonium 210
Maybe I'm wrong but the current media hysteria about Polonium seems to have missed something. The classic Richard Rhodes book 'The making of the atomic bomb' has interesting stuff about polonium. It was and probably still is a vital part of the initiator of all atomic weapons because its a particularly strong alpha emitter ( Its such a strong emitter that it ionises the air around it so permanently appears to glow blue). Particularly relevant though is the paragraph about shipping the Polonium to Los Alamos on page 579.

 'Thomas shipped the Po on platinum foil in sealed containers, but another nasty characteristic of polonium caused shipping troubles; for reasons never satisfactorily explained by experiment, the metal migrates from place to place and can quickly contaminate large areas. 'This isotope has been observed to migrate upstream against a current of air,' notes a postwar British report on polonium, 'and to translocate under conditions where it would appear to be doing so of its own accord.' Chemists at Los Alamos learned to look for it embedded in the walls of the shipping containers when Thomas's shipments came up short.'   

This explains why polonium keeps getting found in unexpected places. But it does also suggest Litvinenko might have been trying to sell some polonium, but was unaware of its strange properties and a bit too much migrated into his body. 

8 Dec 06

I got this reply:

I must say I doubt Litvinenko was trying to sell the stuff – traces
of the Po are all over the tracks of the people he met prior to his
poisoning. His own movements only show contamination after the
poisoning. 

So he was buying not selling!

If someone in Russia intended to occasion an untraceable &
unexplainable death they certainly miscalculated as the alpha
emissions are being found to trace people's journeys in detail.

More fun Po facts: Irene Joliot-Curie, daughter of the famous Mme
Curie, died of leukemia ascribed to the explosion of a container of
Po in her lab. Given that 1g of Po will spontaneously heat to 500 deg
C due to it's radioactivity and is also prone to evaporation (half a
sample in ambient conditions evaporates in 72 hours), one wonders how
anyone would set about transporting it other than under Manhattan-
project circumstances.

Back to theories regarding devious Russians: before presenting at 
hospital it's rumoured that Litvinenko attempted self-medicating, & 
either then or in hospital took Prussian blue as an antidote to 
suspected thallium poisoning. He was also checked with a Geiger
counter because he showed symptoms consistent with radiation
sickness, but this gave negative results due to his contamination
being internal & with an alpha-emitter (thus absorbed by the body).
 If Litvinenko knew he'd been near Po I'm sure he could have found
some way to suggest testing/treatment for it, given there was already
a theory he'd been exposed to an exotic toxin. Not that treatment 
would likely have done much good.