robdwoods - 6:25 pm on Jan 24, 2012 (gmt 0)
I can see how one ad justified top left could trigger the change depending on the other content on the page. Top left is really the prime area every single user is going to look. If you look at almost any heat map of a page or even the Google screen resolution tool the focus of users is virtually always top left. I suspect with this change it's not just the number of ads above the fold but their placement on the page as well. I suspect top left ad placement is rated "lower quality" by the algo than a location like right sidebar, that a banner in the header that doesn't force the content down is rated less "spammy" than an ad at the top of the content in the body section of the page which does force down content. I haven't done enough testing yet to confirm it but my strong suspicion is that placement is as much of the change to the algo as is raw number of ads. It would be an interesting test to move that top left ad to another location above the fold and see if there is any change.
Also please note that lower quality and spammy above are in quotes. I certainly don't mean to suggest that any site here has those attributes, but rather that that's the way the algorithm may see them under it's current configuration.