Nah, you're just a typical Russophic, Nazi-loving Finn

The reason that I don't think it's Putin is because it just doesn't make sense.
First of all, these are minor and marginal critics which are being killed, meaning Litvinenko and Politskovaya, and are of no real threat to the government. For God's sake, Litvinenko believed that the Russian government supported al-Qaeda in '98 and had something to do with 9/11 - he was more damaging to his own reputation than to Putin's.
Secondly, I have to believe that the FSB, the successor to the world best and most feared intelligence agency, would have ways of killing people which would be absolutely untrackable. Either untraceable poison or just a convenient motive for a shooting (a simple robbery would do). Killing Politskovaya on Putin's birthday seems like a very bad way to be discrete, hardly the sort of thing you would expect from professionals such as the FSB. Why would they kill him with a high-tech and hard-to-obtain radioactive substance, instead of simpler method?
And thirdly, Putin's not stupid. He knows that he'll be blamed for any killing to his opponents, and that his and Russia's image will suffer. So why do it? It's a similar situation to Lebanon and Pierre Gamayel's assasination: the accussed party simply does not have much to profit by carrying out the murder, and has much to loose.