hyperkik - 4:59 pm on Mar 4, 2011 (gmt 0)
I have a site with two distinct areas of content, one of which has been affected to a significant degree and another of which has not been affected at all (or may have received a slight boost). It's too small a sample size from which to extrapolate, but I can tell you this: The algorithm is considering more than "how many ad units are there" or "what size are the units", and you can trigger the penalty without having an excessive number of units on a page, and even if you have a ton of text on the page.
My impression at this point is that it's the placement of an ad with CSS that's causing the problem - Googlebot can't figure out where that ad unit appears on the page and that is causing the trust to plummet. I'll be doing some testing, albeit probably not as scientific as incrediBill's, to try to determine if my hunch is correct.