ProxyPass rendering issue under IE

I am trying to configure apache ( apache 2.2 ) to allow us to proxy an
external web site through our domain. It seems to be fine under
Firefox, but there are some rendering issues ( all of the images/css
seem to load fine, but the positioning is broken ) when accessing this
under internet explorer. We have done a diff on the HTML source of
the page in IE vs. Firefox's source - they are the same.

Does anyone know what would cause the proxied page to render
differently under IE than Firefox?