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About two weeks ago, at the International Workshop on Hadamard Matrices and Their Applications (RMIT), http://user.gs.rmit.edu.au/asha/iwhma/ Jennifer Seberry presented a proof, based on counting arguments http://user.gs.rmit.edu.au/asha/iwhma/html/Wednesday/JenniferSeberry.pdf, but that proof was quickly found to be incomplete. As far as I know, she is currently working on improving the proof. A special volume of the Australasian Journal of Combinatorics http://user.gs.rmit.edu.au/asha/iwhma/html/ajc.html is forthcoming, and with any luck, things will become much clearer at that point.