Hopefully sometime this year, although I won't need one because I need it now or the 600 will suffice. And that may be why there're holding off. People are still buying the 600.
I'm very interested in how third parties will respond. Soon enough Canon will have all their flashes with wireless functionality, and Nikon will mimic the functionality in their flashes. What will Pocket Wizards, and the like do? I still see a need to trigger my alien bees or another non-Canon non-Nikon flash/strobe. But third party triggers may get crushed.