To be honest, I'm not sure. At first I thought pages were only falling back to positions somewhere around 100-200. But then I realized that the keyword tracking system didn't account for the fact that it was actually a different page on the site that was being returned in that position.
In other words take one page about Super Widgets. It used to rank in the top five. Then the keyword tracker would report my site ranked 175 for that phrase, however it was actually for a different page on my site that happened to mention Super Widgets. The page which is dedicated to Super Widgets was actually much, much further down in the results.
I can say that there wasn't much variation at the top. I don't know about the bottom though.... as far as G referral traffic goes there isn't much difference between being ranked 50 and being ranked 950.