This gives you a new event (onDOMReady).

Object.extend(Event, {
  _domReady : function() {
    if (arguments.callee.done) return;
    arguments.callee.done = true;

    if (this._timer)  clearInterval(this._timer);
    
    this._readyCallbacks.each(function(f) { f() });
    this._readyCallbacks = null;
},
  onDOMReady : function(f) {
    if (!this._readyCallbacks) {
      var domReady = this._domReady.bind(this);
      
      if (document.addEventListener)
        document.addEventListener("DOMContentLoaded", domReady, false);
        
        /*@cc_on @*/
        /*@if (@_win32)
            document.write("<script id=__ie_onload defer src=javascript:void(0)><\/script>");
            document.getElementById("__ie_onload").onreadystatechange = function() {
                if (this.readyState == "complete") domReady(); 
            };
        /*@end @*/
        
        if (/WebKit/i.test(navigator.userAgent)) { 
          this._timer = setInterval(function() {
            if (/loaded|complete/.test(document.readyState)) domReady(); 
          }, 10);
        }
        
        Event.observe(window, 'load', domReady);
        Event._readyCallbacks =  [];
    }
    Event._readyCallbacks.push(f);
  }
});

Example:

Event.onDOMReady(function(){  
alert('DOM is loaded!');  
});