How do Google, say, use bounce rates - they do have bounce rates for folks using Analytics, and I guess they could guess some form of bounce rate by analysing search clicks but they'd be missing data for lots of pages and lots of visits on pages they could get a rate on. So, do you have any insight in to how they or Bing or whoever do it in practice; or is it just a guess?
SEOmoz ranking factors, http://www.seomoz.org/article/search-ranking-factors#metrics..., (and other pages there) suggest it's based on clicks on SERPs that follow after a visit to your site. Is that the/a known method or just the most speculated method.
Seems it could be potentially abused, so for example the "are you sure you want to leave this page" (onbeforeunload) type warnings will clearly elongate the page visit duration.