They're out there
We've been told by a couple of places, that the new EF 600 f/4L IS II is out there. Loaner copies of the lens have been sent to various manufacturers of accessories for measurement and that sort of thing. Multiple reports say they are fully functional production versions of the lens.
Lots of initial praise for the lens, especially how light it feels in comparison to the first IS version.
A very good sign indeed.