Did you read that notice from google, too? “We are adapting our ranking algorithm a bit – it concerns only 1% of all searches, and if you listened to our previous adsense advise you are now screwed” … well, that is not verbatim, but it was something along these lines.
What has happened? Actually not necessarily a bad thing. Google is taking into account what readers will see above the fold (i.e. what you see without scrolling) – if you plaster that space with ads only and make it hard for your readers to find the actual content that they have come for, that creates a bad user experience and bad user experience is the red flag for google. You will eventually loose rankings.
But what about the former advise to put large ads at the beginning of your text – well that has somewhat been revised, you are still encouraged to find spots with good CTR and that still applies very much to that space above the fold, but you should take care that there is actually some content and not only ads. This is especially important if your site has a two sidebars design with ads on both sidebars. When you then put another ad at the beginning of your content you might very well already be over the top. I have not yet read much about whether this applies to all kinds of ads or only to adsense, but my personal bet is that google has become smart enough to recognize most ads and thus this will apply to any over full ads page…