It looks like the menu ul isn't inside the topNavigation
it goes: header, ul, script, topNav
seems like the ul and script should be inside the topNav
The gap seems to have something to do with the differences in how the browsers handle the <p> tag. If I remove the <p> tag from the first text block the gap disappears as well.
It actually has the gap in IE8, safari, and FF. Previous versions of IE show without the gap, so the better question might be why don't earlier versions of IE have the gap which it seems show in modern browsers.