dertyfern - 9:30 pm on May 15, 2010 (gmt 0)
I've seen serps served up in somewhat random order with high frequency as of late.
A simple clearing of all cookies and cache and resubmitting a query is showing results in differing orders.
This seems to apply to three keyword searches and longer. One and two word searches seem to be exempt, which may be one reason why primary positions are still strong but traffic has dropped off.