Excellent post MsHuggys.
To back it up, I have 336 x 280 top and bottom of many of my pages. The one above the fold has a CTR about twice the one at the bottom of the page, and an even better eCPM. No question in my mind, above the fold works better.
But, even allowing for the Google blurb, how can they be sure that ads are above the fold? There are some clever tricks which can be used to disguise the exact position content appears on a page. No algo will be clever enough to beat the really creative designer.
Which makes me think that only "simply designed" pages which clearly display ads ATF will be included.