P.P.S. With the DOM Level 2 Event Model, you can assign as many listeners to a DOM element as you like, but IE doesn't support it. Always the life of the party, IE has its own proprietary API. So most people just use the DOM Level 0 Model (what's demonstrated in the previous posts).
Employing a library such as jQuery or Prototype hides all (or at least most) of the event handling browser differences from your code.