i think that Observer pattern is always implemented as "static" to be called from any scope, for example:
<?php
class Observer {
     //attach here the caller's scope
     private static $stackTrace = array( );
     public static function wasTriggeredOnce() {
          //...if is not in the stack, then:
          self::$stackTrace[] = max( debug_backtrace( ) );
     }
}

class YourFramework {

     public function launchPlatform() {

          //could not let user to launch application twice!
          Observer::wasTriggeredOnce();
     }
}
//cause is static
Observer::hereIsnoNeedToInstantiateAgain(); 
?>

The most obvious storage type for observers (stored in attach()) is array. In order to be able to detach attached objects, you should be able to identify it inside observer storage.

I suggest you attach observers this way:

$this->_observers[spl_object_hash($observer)] = $observer;

so you can detach it later if you need:

unset($this->_observers[spl_object_hash($observer)]);