Robert_Charlton - 11:43 pm on Nov 21, 2010 (gmt 0)

The bounce rate for Search Engine Traffic DROPPED from around 65% to around 12% on October 19th.

Agreed, this seems almost too good to be true.

Grasping at some straws, there has been discussion in the Traffic Shaping thread that fits some of what you're describing, but nothing to the degree you cite above....

Google & Traffic Shaping - a hidden method to the quality madness?
http://www.webmasterworld.com/google/4222996.htm [webmasterworld.com]

The OP in the traffic shaping discussion notes a 20% increase in conversions with no increase in traffic, but with a different set of referrals. Another poster cited 10% decrease in bounce rates. Have your conversions been affected?

Does anything in that thread suggest similarities to what's happening with your traffic?