dataguy - 6:35 pm on May 31, 2011 (gmt 0)
This is based on traffic patterns, not necessarily on uptick or down-tick in traffic, though. But it's been spot-on for signalling every major change in ranking for almost 5 years now.
But incidentally, I'm seeing about a 10% uptick in traffic on my heavily Pandalized site. It seems to have been backed down since it started, but I haven't seen this since Feb. 23rd so maybe it's a sign of good things to come.