[geon]

I think YouTube borrowed the menu-behind-the-content-when-it-slides-to-the-right from Facebook. Anyway, that's where I first saw it.

Design-wise, it looks pretty much like the new mobile website.

[christensen_emc]

The menu behind the content UI Pattern is actually one of the suggested UI patterns in the official Android style guide.

[geon]

Ah. Found it (they call it "drawers").

http://developer.android.com/design/patterns/actionbar.html

[Zev]

Do you know when this UI pattern was added to the style guide? Because, FWIW, webOS had something similar with sliding, over a year ago, in their Maps app.