No jQuery solution :
var list = document.getElementsByTagName('ul')[0],
items = list.getElementsByTagName('li'),
i = items.length;
while (i--) list.appendChild(items[i]);
Here is a demo : http://jsfiddle.net/wared/tJaJ9/.
Based on cookie monster's suggestion :
var list = document.getElementsByTagName('ul')[0],
i = list.children.length;
while (i--) list.appendChild(list.children[i]);
Just for fun :
var list = document.getElementsByTagName('ul')[0],
items = Array.prototype.slice.call(list.children);
while (items.length) list.appendChild(items.pop());
A jQuery one :
$('ul').append($('li').get().reverse());