OK, today's the day. It's been "rolled out."
Color me skeptical so far. The pre-done themes are very "modern", in the Windows 8 sense (for good or ill depending on your taste), right down to the inscrutable icons barely noticeable on the screen. Not quite mystery-meat navigation, but not real obvious to new visitors either, IMO. I'm also finding it hard to recreate the simple gallery-centric layout I have been using on my "legacy" SmugMug site this past year. In fact, my existing galleries and collections don't seem to be recognized when I enable a gallery "block" - I needed to drag a folders block onto the home page to display them. Probably a semantic change I need to read up on, but it appears I'll have to revisit my photo organization scheme.
I think there's a lot of potential for making a more dramatic-looking website with pre-done themes (the old ones were definitely "dated"), but I'm going to have to completely rethink my approach to the site and that's always a pain. The good news, though, is that they let you try out the new layout tools and themes indefinitely without fully committing (as far as your visitors are concerned), so you can take your time. Once you do switch over, though, it's one way.