Simple JQuery UID Plugin

UID's can be tricky to make, but here's a simple one for discussion.

I'm posting this to see if anyone can make it a little safer to prevent collisions and/or improve the algorithm. Any thoughts? The prefix allow you to create uids you can select with a prefix on. The random string will give you a fairly large but not unmanageable string. We'll also return the JQuery instance so we can continue our chain.

(function($){  
 $.fn.uid = function(prefix) {  
            if (!prefix) {
                prefix = "uid";
            }
     var generate = function() {
  var dt = new Date().getMilliseconds();
  var num = Math.random();
  var rnd = Math.round(num*100000);
  return prefix+dt+rnd;
     };
  return this.each(function() {  
          this.id = generate();
   return $;
  });  
 };  
})(jQuery);

Usage:
$().ready(function() {
  $('div').uid().css('background-color','blue');
});

Oui? Non?