according to this roundup of SES Chicago http://searchenginewatch.com/3641509
QuoteOhye mentioned that the Page Speed calculations you see in Webmaster Tools are based on data from the Google Toolbar.
I think thats the first time, if accurately reported, I've seen Google admit that toolbar data is specifically used in the rankings? Assuming you accept that load time is part of Cafeeine, of course.
Actually that information has been available on the Webmaster Tools site since they startes publishing opad times
aaah. I really should pay attention to that sometimes!