This looks like a great redesign overall. I'm still not entirely sold on how sheets are any better than breadcrumbs, other than being a different UX implementation of the same concept. But I'll adopt a wait-and-see attitude. Playing around with it is definitely needed for something like this.

The one thing I will say though: every link better be "openable" in a new tab. While I love that you're helping me focus on one thing on the page, we're multitasking beasts at heart. If I can't Cmd-Click anything in that interface I'm going to be quite disappointed. But if you're using HTML5's pushState I'm assuming you have that already taken care of ;)