Last night Ant stayed up until about 4am trying to find the CSS solution to a problem both of us were encountering on our blogs. The problem being that if main content side of the page is shorter than the sidebar side, the sidebar moves over onto the left at this point.
We'd tried all sorts of things using float and other such things, but the sidebar would always drop to the bottom right below all the content. He realised that this was to do with the overall widths being greater than 100% (due to paddings and margins). Setting the width of the sidebar (or removing paddings and margins) seems to solve this problem. It meant that other things didn't look quite right. Like the vertical bar that used to separate the sidebar and the blog content.
I've been playing to get something that looks good... still not 100% sure of it though... what do you think?