handle various controllers from a single pre-dispatch

observer.php

<?php
class Observer
{
  public function handleControllerActionPredispatch(Varien_Event_Observer $observer)
  {
    $controllerAction = $observer->getEvent()->getControllerAction();
    
    if ($controllerAction instanceOf Mage_Checkout_OnepageController) {
      
      // currently handling a onepage checkout route
      
    }
  }
}