petit is right. IE handles the box model differently (dare I say, poorly). Sometimes is has to do with the has_layout property flag (don’t worry about it)… and sometimes it has to do with the left and right borders and padding.
More often than not.. when your sidebar gets pushed to the bottom of other content in IE… it is because IE thinks the sidebar is TOO WIDE. Find images or CSS-defined widths that may be wider than others, or just pushing the limits of the space the sidebar is supposed to fit into… and then try tweaking them to be thinner and see if this helps.
Also, as a general tip, make the sidebar about 5px thinner than it is allowed to be in that space… allowing for some “slop” can cover a multitude of sins.