function adaptMenuPositioning() {
	if (Element.getLayout('navigation').get('height') > document.viewport.getHeight()) {
		$('navigation').setStyle({position:'static'});
	}
	else {
		$('navigation').setStyle({position:'fixed'});
	}
}

Event.observe(document, 'dom:loaded', adaptMenuPositioning);
Event.observe(document.onresize ? document : window, 'resize', adaptMenuPositioning);