Maybe they released this by location, but I've seen this new design for the past few weeks. It's nice that the logo doesn't have jaggies now, like it still does here: https://gist.github.com/
Edit: Seems that's actually a Firefox issue. Compare Chrome 21 (top) vs Firefox 18 (bottom): http://imgur.com/5XSRH
Seems like they have a "new design" every few months now.
Continuous deployment. Chrome is on version 23. IE is on version 10.