true_INFP - 12:33 pm on May 2, 2009 (gmt 0)

My humble opinion is that the cause is:

* Google silently taking a bigger portion of publisher's earnings to meet the market expectations.

There is no reason to believe the cause is ad blindness (although it certainly plays some minor role), because there have been at least two occasions where far too many advertisers saw an "inexplicable" and abrupt drop by ~50% during a very short period (a few days).