From the always-interesting Canonical Design blog: Introducing Overlay Scrollbars in Unity. (Who wouldn't love to see an Apple Design blog? It's fun to follow along as some serious, talented people rethink stuff we use everyday…)
Take a look at the video - and stick with it all the way through. At first I didn't like it but now I'm a fan. I like that they've expanded usable screen real estate by getting rid of the everpresent thumb, but kept a subtle visual indicator that there's more content - there's a small ~2px bar that scales like current thumbs do.
My only concern would be that the little mini-thumb is too small and it will sometimes not be obvious that you can scroll. I think this happens on iOS, too - maybe it's not such a big deal.