Somehow I missed this information when it came out in 2009 (or perhaps a little earlier, as the original book by was in German) - conjecture that Gauguin was the one to slice Van Gogh's ear based on ten years of research into the matter by two German academics, Hans Kaufmann and Rita Wildegans, who then wrote "Van Gogh's Ear: Paul Gauguin and the Pact of Silence.
An article on the book back in May 2009 - and the gist of the findings:
There is a current article by Adam Gopnik in the Jan. 4, 2010 New Yorker,
Van Gogh's Ear, the Christmas Eve that changed Modern Art - that you can tell from the title goes into the impact of this on artists to come. The article also goes on at length about van Gogh and Gauguin as individual artists. The full text of Gopnik's is only available to subscribers (unless you pay a fee) but it is probably still on the news stands if you're are interested (and have just the right newstand or library nearby).
The abstract here doesn't go into the details..