First, a disclosure: I love the idea of moving the whole website over to reveal the navigation tray below. I think it's really slick as well as unique in the world of desktop websites, and in my particular case (http://forr.st/~3mF) it helps remove clutter from above the fold so that the focus can be on the slideshow of my recent projects.
That said, requiring my user to click once before seeing the possible places to go makes them take an extra, unnecessary step. This creates some level of friction for a user trying to move around a new website.
So I guess I would like to open up a discussion here about the benefits and downsides to implementing this kind of navigation pattern in a desktop site (a fad that is spreading to quite a few desktop sites in one form or another), what the possible effects are on usability, efficiency, and visual interest, as well as what helpful actions might be taken by us designers to aid users unfamiliar with this pattern.
Thank you for your participation, and I hope we can all learn something from this! :)