I seem to remember acquiring my 5D MkII around Christmas so that kind of announcement would tie with the 5D MKII. If this rumour is true then the design of the MkIII would have to finalised, the production line in place and ready to roll, because the sheer amount of product required to satisfy demand is enormous. I wonder what the time is for a camera to begin its life, to it being shipped and available on the shelves, with a new camera like the 5D MkIII with high initial demand it wouldn't be unreasonable for a 2 month build up a stock to satisfy that. The 5D MkII took Canon a little by surprise when it was unable to keep up, so is the 5D MkIII already in production?
Of course all of this assumes an October announcement!