Around the weekend of Turing's centenary Prof. Jack Copeland, a renowned Turing scholar, author and digital archivist raised the notion that Turing's death by cyanide poisoning may have been an accident and that the coroner had insufficient evidence to state it was suicide. Copeland's theory not surprisingly gathered a lot of publicity, including this report by the BBC.
The Alan Turing Century blog brought to my attention that the makers of the documentary film Codebreaker have carefully refuted Prof. Copeland's arguments point-by-point in a blog post titled: "Turing Committed Suicide: Case Closed."
Personally, I believe that, tragically, Turing committed suicide. Of course there is a third theory, namely that Turing was murdered by the secret services to prevent him giving secrets to the Russians. A colleague of mine Dr. Gary Tee has claimed that he knows "Turing was murdered," but he's never elaborated on this. Certainly it's easy to imagine a scenario in which paranoid cold-war spooks decide that Turing's death would benefit national security.