function fixHeight()
{
	var y = $('main_container').cumulativeOffset().top; 
	if (document.viewport.getHeight()<Element.getHeight(document.body))
	{
		$('main_container').setStyle({height:'auto'});
	}
	else
	{
		var h = (document.viewport.getHeight() - y - 2).toString() + "px";
		$('main_container').setStyle({height:h});
	}
}

Element.observe(window, "resize", fixHeight);
Element.observe(document, "dom:loaded", fixHeight);