nods, seems pretty similar to MW's CategoryTree? It's good to have, but it could be much more awesome with attention. Especially if tags cause complex trees, it'll allow smoother content discovery with a more optimized setup.
What's the plan for navigation back up the tree? If the top level tags are always exposed things could get very messy (a bunch of layers, and possibly multiple paths back to them, since unlike a file tree each page can have multiple direct ancestors, unless you only include parent/child relations.), and if not you need another way to step back up.
Edit: I asked about the name of the UI I'm talking about on StackExchange. Some possibly helpful comments there.
Edit2: It seems like it's allowing multiple parents that breaks the normal navigation things. Maybe some hybrid would work.. I'll do a mockup of what I'm imagining at some point. The UX people are not familiar with it. Probably complex+no third party drop in, so probably quite far down the wish list, even if it would be awesome.