OK I haven't read the 10+pages of posts on this thread, but Khan's identity was a secret? IMDB.com had Cumberbatch playing Khan from the get-go. This was later changed to John Harrison, but IMO the cat was out of the bag.
IMDB is not ever to be trusted as a source of info for forthcoming movies. This was the rare time when someone posted a rumour there that turned out to be a fact.
Cumberbatch was never officially revealed by Bad Robot to be playing Khan prior to the movie's release. In fact, for ten months after he was cast nothing was given in regards to who he was playing, then it was revealed to be "John Harrison."
Everyone guessed he was Khan anyway, but Bad Robot attempted to keep it a secret regardless.