Marcia - 10:42 am on May 7, 2008 (gmt 0)

So then, it starts to sound like they're reserving the #4 spot for Universal Search and rotating random sites into the spot in between, selected by some sort of undetermined criteria for testing purposes.

Pure speculation: could it be demarcation by human review of sites that appear to be "worthy" of a high quality score, and rotating them in and out to test, to see if CTR and bounce rate corroborate it?

[edited by: Marcia at 10:44 am (utc) on May 7, 2008]