If Google is trying to use this as a ranking signal, they will have to deal with some complications:
-- A lot of people will rapidly open a series of tabbed results by repeatedly jumping back to the SERPs page. They won't even look at any of the pages until they finish opening a series of tabs. This makes it appear that rapid bouncebacks are occuring when they aren't.
-- For many sites a high percentage of the Long-tail traffic is mis-matched to the content of the landing page. The resulting quick bouncebacks are Google's fault rather than the website's fault.
Unless Google can find a way to take account of these complications, I don't think they can use bouncebacks or bounce rates as reliable ranking signals.