Well, it’s not that mysterious. Like the 1D Mark II N, this will probably be a minor update: increase in megapixels, maybe a better LCD screen, a better cache for burst shots, etc. It was probably to be a surprise at PMA, but now everyone will have to just pretend like they didn’t know. Awkward!
A commenter points out that the price is listed at €4240, or almost $5600. That’s a bit of a premium over the 1D’s competition, so unless that N brings some serious features with it, there may not be too much excitement.