The Polonium had to come from a sophisticated source. Either it was connected to Putin, or it wanted to appear connected to Putin. I don't have a dog in this fight, but it seems like there are an awful lot of people who eagerly jumped on the anti-Putin bandwagon.
Occam's Razor time - the simplest explanation is usually the correct one. What does Russia gain from killing Litvinenko? Putin not only silences a critic, he also sends a powerful message to those who object to his regime based abroad. That, coupled with the radioactive material, which contrary to what you may hear, is not easy to get - points the finger at a state, namely Russia.
The idea that Berezovsky would poison his own colleague in the attempt to discredit Russia would be extremely expensive and pointless.
Ivan