I am sure that everyone has heard about theformer KGB agent in London killed by a lethal dose of Po-210 (Polonium-210).

First, what is a lethal dose? I looked up the numbers: 1.17*10^-7 grams is guaranteed to cause death- (This means it is more toxic than botulin toxin) given the density of polonium, a cube of the metal would only need to be 20 microns on a side to be the lethal dose- that is the width of a fine hair... The stuff has a decay halflife of ~140 days... and a biological halflife ( A measure how fast it leaves the body) of 50-60 days. So a nearly invisible amount will kill you- it kills you in a horrible way, radioactive cooking of your organs- it is a STRONG alpha particle emitter- each alpha particle emitted carries an energy of 5.4 million electron volts- this energy is deposited in the cells it hits, doing fatal damage.

Polonium is found in Uranium ore in minute amounts- mostly it is made by bombarding an isotope of bismuth with neutrons, and then cook the polonium out of the bismuth... to make it requires quite an infrastructure- essentially you need a nuclear program. According to wikipedia, only about 100 grams of the stuff is made per year.

Given its short half life, building up a stockpile of the stuff is not very practical.
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The reports say that polonium has been found at the victims home and a sushi bar he ate at- perhaps he left traces after being poisoned, but one odd aspect of polonium is that an exposed sample of it evaporates into the air fairly quickly- perhaps his assassin was carrying the material around, following the victim....

It has been suggested that Putin arranged for this man to be killed in this manner, but this does not make sense to me- if you are going to kill a political foe, why do it with a poison that points directly at you? Joe Schmoe terrorist is not going to be able to make the stuff... Polonium points to the involvement of a nuclear capable state's involvement. The use of polonium ensures that Moscow will be the prime, if not only, suspect.

This seems like a setup to me...