According to this article Larry Page said
"We're..acting in the interest of users..that half that goes up will be happy but not say anything"
I've always been a fan of pseudo-mathematics!
Let's say you pull the top 100 in the serps, and sink them to oblivion (e.g. into the mid position.) Then the new top half of the serps goes up by 100 and the bottom half, er, down by 100.
Give or take a few decimal places, what's gone up, has indeed gone down! 50/50, seems absolutely fair to me.
Pure genius, and up goes the Adwords revenue too, because the decent sites are now off the radar!
p.s. I'm sure Larry's maths is better than mine, maybe he was having an off day :)