I've just tried the site in Chrome, Firefox and IE 10 Release Preview, and the only difference I can see is that the little arrow (^) when you hover over a dropdown menu item doesn't show in IE. Other than that, they look and behave the same for me.
I checked the site on IE9, Chrome(latest) and Firefox(latest). Seems that it is working great. Don't worry about older browsers like IE6. No web design company supports IE6 instead they would put a warning for it saying that they should upgrade.
I found a good warning script, check it: http://code.google.com/p/ie6-upgrade-warning/
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