A) Believe he was poisoned, or
B) Believe he is working with a vast shadow conspiracy of Islamic terrorists and was accidentally poisoned during his routine maintenance of "suitcase nukes."
Well, we know this much: he was poisoned.
We also know that (a) it was enough of a very expensive poison to kill him a gazillion times over; (b) he left large amounts of radioactive byproducts all over the damn place in a fashion apparently inconsistent with having actually ingested the poison, thus leaving the how of the poisoning an open question; (c) he had converted to Islam (funeral rites were held at a London mosque, minus his body, which was too damn radioactive), raising still further questions; (d) he was a close personal buddy of an exiled Chechen leader who has been tied to nuclear smuggling by non-Russian sources; and (e) his patron was a Russian robber baron with very close ties to Chechen rebels, Ukranian weapons smugglers, and other sterling citizens.
It's a pity Robert Ludlum is dead; he could have written a 900-page masterpiece with about one plot twist per page from this...
It's my belief that as the deceased himself said it was a KGB hit. Assassinations - hits, rather - are not about the death. A gunshot to the head is efficient and cheap. Poisoning with a rare radioactive substance sends a message. Mob hitmen used to use chainsaws, cyanide, you name it - because it wasn't just about killing the guy.
Suicide is another option. It's unlikely and would require Litvinenko to want to have discredited somebody, but it is possible. That might explain how it happened.
I'm willing to entertain more than a few theories, but the suitcase nukes theory is ridiculous.