Interesting idea. I did something similar (although with dynamic page-loading) for a client a few months ago: http://ericschmid.ch/
Cool idea, but there's no y scrolling.
I think it's a cool transitional interface, but it's a bit misleading. There are no actual page loads happening, which puts all of the burden up front with loading the full website and all of its subpages at once. Also, what determines the order of the stack? I thought at first, it was a page history log, but that proved to be false.
Interesting idea, but when scaled, seems unreasonable.
Very slick, but I think the actual page stack confuses things. The three stacks seem to relate to a navigation column, but the columns don't really relate to one another in a clear and obvious way. And that relation doesn't scale well in either direction and would presumably break down entirely on smaller displays when the navigation items are forced into a single column.
I think this would work better without the stack.