i'm having the same problem. I've isolated the problem to the fact that IE8 by default, is loading all sites in Compatibility View. I've seen threads that you can set a <meta> element, to force IE to use standards view (IE8), but does not seem to work for me ... though if I force IE into always using Standards mode, everything looks good.
fyi, if the IE8 compatibility view settings are set to "display all sites in compatibility view", e.g. that option is checked, or if a site is configured to always use compatibility view, then your gwt/gxt components may not display correctly.