I read up on the information and it looks like you can include a Meta Tag to force compatibility users to display in "Standards" (IE8) mode but not the other way around (which is what you need).
I would just show a graphic like this:
http://www.lasos.org.uk/images/skin/ie8compatmode.gif
in a lightbox (http://www.huddletogether.com/projects/lightbox2/). Then when they are in compatibility mode or in another browser, it is set to display:none in the css.