Polonium 210 is interesting stuff. As noted in a variety of places on the web, it is entirely artificial — it doesn't occur naturally, but has to be created by irradiating bismuth in a nuclear reactor or particle accelerator — and it has a half life of 138 days, decaying via alpha emission. To do any damage, it needs to be up close and personal, inside the victim, because alpha particles are absorbed very rapidly: but the biological damage they cause is much more severe than gamma radiation, neutrons, or beta radiation, precisely because all their energy gets dumped into bodily tissues promptly, rather than most of it zipping right through the victim and dissipating harmlessly in mid-air.
If, as talking heads have said, only hundreds of grams of polonium-210 are manufactured per annum, the conclusion that someone very powerful wanted Litvinenko to die in a very nasty way, perhaps pour encourager les autres, seems inescapable. Not that it was ever likely that he ate bad sushi, mind. It's worrying that Daniel Davies' conclusion that "[a]ll of the main suspects are simply too geopolitically important in one way or another to ever be charged with or punished for anything as simple as murder" is almost certainly correct. I hope that this doesn't initiate the era of radiological terrorism, but we'll see, won't we?