Blog site: cdlmn.org/CDL
I get a bad presentation in two cases: a) when older posts are called by selecting "Previous Entries" or when a category "Marriage" is selected (this contains older posts).
Using the default theme, the white part of the background image goes away and the sidebar is moved below ALL posts.
I believe that I have a "vanilla" default theme, straight from unzipping. I also have a modified "WordPress Default 1.5 by Michael Heilemann" which I added some CSS code. The problem happens with both.