The difference you observe is the difference in the default margin and padding for the body element between browsers. In the case of IE and FF, it is only margin, but some other browsers have default padding for the body as well. So, what you could do is remove the individual style for the div, and zero out margin and padding for the body:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled Document</title>
<script src="Scripts/AC_RunActiveContent.js" type="text/javascript"></script>
</head>
<style type="text/css">
body {
margin:0;
padding:0;
}
</style>
<body>
<div>
<script type="text/javascript">
AC_FL_RunContent( 'codebase','http://download.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=9,0,28,0','width','740','height','110','src','ddth_740x110','quality','high','pluginspage','http://www.adobe.com/shockwave/download/download.cgi?P1_Prod_Version=ShockwaveFlash','movie','ddth_740x110' ); //end AC code
</script><noscript><object classid="clsid:D27CDB6E-AE6D-11cf-96B8-444553540000" codebase="http://download.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=9,0,28,0" width="740" height="110">
<param name="movie" value="ddth_740x110.swf" />
<param name="quality" value="high" />
<embed src="ddth_740x110.swf" quality="high" pluginspage="http://www.adobe.com/shockwave/download/download.cgi?P1_Prod_Version=ShockwaveFlash" type="application/x-shockwave-flash" width="400" height="60"></embed>
</object></noscript>
</div>
</body>
</html>