Does Google consider bounce rate or something similar in ranking sites?

Background: here at Stack Exchange we noticed that the latest Google algorithm changes resulted in about a 20% dip in traffic to Server Fault (and a much smaller dip in traffic to Super User). Stack Overflow traffic was not affected.

There was an article on WebProNews which hypothesized that bounce rate might be a ranking signal in Google's latest Panda update.

According to Google Analytics, these are our bounce rates over the last month:

Site           Bounce Rate Avg Time on Site
-------------  ----------- ----------------
SuperUser      84.67%      01:16
ServerFault    83.76%      00:53
Stack Overflow 63.63%      04:12

Now, technically, Google has no way to know the bounce rate. If you go to Google, search for something, and click on the first result, Google can't tell the difference between:

• a user who turns off their computer
• a user who goes to a completely different web site
• a user who spends hours clicking around on the website they landed on

What Google does know is how long it takes the user to come back to Google and do another search. According to the book In The Plex (page 47), Google distinguishes between what they call "short clicks" and "long clicks":

• A short click is a search where the user quickly comes back to Google and does another search. Google interprets this as a signal that the first search results were unsatisfactory.
• A long click is a search where the user doesn't search again for a long time.

The book says that Google uses this information internally, to judge the quality of their own algorithms. It also said that short click data in which someone retypes a slight variation of the search is used to fuel the "Did you mean...?" spell checking algorithm.

So, my hypothesis is that Google has recently decided to use long click rates as a signal of a high quality site. Does anyone have any evidence of this? Have you seen any high-bounce-rate sites which lost traffic (or vice-versa)?