I haven't verified this in any other version of IE, but IE6 has a weird bug that breaks pages in spectacular fashion. Basically, pages with an apparently innocuous CSS rule (background-repeat:repeat-y;) end up completely garbled if they are contained in a frame.
Fixes? Well, allegedly you can fix it by adding a repeat rule for every contained element. That doesn't seem like the way to cleaner simpler markup, however, so I fixed it on the site I was working on by using a very wide background image and not specifying the background-repeat property. This means, of course, that somebody with a very wide monitor would see wrapping... Ugh.