jQuery.fn.swap = function(b){
b = jQuery(b)[0];
var a = this[0];
var t = a.parentNode.insertBefore(document.createTextNode(''), a);
b.parentNode.insertBefore(a, b);
t.parentNode.insertBefore(b, t);
t.parentNode.removeChild(t);
return this;
};
and use it like this:
$('#div1').swap('#div2');
if you don't want to use jQuery you could easily adapt the function.