In AS 3.0, if you do want to pass params in your Events you can create your own custom events:

update 12.08.08:
I have updated the class to be able to carry any kind of parameters, in an array, making it more flexible.

PLAIN TEXT
Actionscript:
  1. package com.events {
  2.    
  3.     /**
  4.     * ...CustomEvent class
  5.     * to attach params to dispatch event in arg:Array
  6.     * @author Sim   15/03/2008 22:56
  7.     * @usage    import com.events.CustomEvent;
  8.     *       var myCustomEvnt = new CustomEvent("EventName",parameterArray);
  9.                 dispatchEvent(myCustomEvnt);
  10.                 // OR simpler:
  11.                 dispatchEvent(new CustomEvent("EventName",parameterArray));
  12.                
  13.     */
  14.     import flash.events.Event;
  15.    
  16.     public class CustomEvent extends  Event {
  17.        
  18.         public var arg:*;
  19.        
  20.         public function CustomEvent( type:String, ... a:*) {
  21.             super( type );
  22.             arg = a;
  23.         }
  24.         override public function clone():Event{
  25.             return new CustomEvent( type, arg );
  26.         }
  27.     }
  28. }
  29.  
  30. /*_______you then dispatch this like this______*/
  31.  
  32. //you would have to import your custom event
  33. import com.events.CustomEvent;
  34. var myCustomEvnt = new CustomEvent("EventName","1","2","3");
  35.                 dispatchEvent(myCustomEvnt);
  36.  
  37. //again you would need to import your custom event
  38. import com.events.CustomEvent;
  39. hboxvar.addEventListener( "EventName", dosomething );
  40.  
  41. private function dosomething( event:CustomEvent ):Void{
  42.     trace( event.arg) // output: 1,2,3
  43. }

another interesting approach can be found here, where flep assigns multiple event types to one custom event, to have more variety in eventHandling.