Marissa Mayer director of Consumer Web Products at Google, spoke at BayCHI on Tuesday evening. Some interesting points she made:
(a) When she first got involved in User Interface Design at Google, she was asked not to spend more than one day a week working on the interface! Also, she was asked not to give her opinions, but to provide data. So right from the beginning, design at Google has been very data driven.
(b) They do a lot of usability testing (mostly discount tests), both task-based and think aloud. From what I understood they do look at statistical significance of such test results.