viewport_h=getViewport();

function getViewport()
{  viewport_h_=600;
	if(self.innerHeight) //tutti tranne Explorer
   {   viewport_h_=self.innerHeight;
   }
   else if(document.documentElement && document.documentElement.clientHeight) 
   {  viewport_h_=document.documentElement.clientHeight; //ie7
   }
   else if(document.documentElement && document.documentElement.offsetHeight) 
   {  viewport_h_=document.documentElement.offsetHeight; //ie5 e 6
   }
   else if(document.body)
   {  viewport_h_=document.body.clientHeight;
   }
	return viewport_h_;
}

function centra_verticalmente(_viewport_h, _div_h)
{	_mt=Math.floor((_viewport_h-_div_h)/2);
	if(_mt>0)
	{	document.write('<style type="text/css">');
		document.write('div#main {');
		document.write('	margin-top: '+_mt+'px;');
		document.write('}');
		document.write('</style>');
	}
}