For those interested I've sent a patch for implementing a new notification mechanism.
It's available here:

http://jira.xwiki.org/jira/browse/XWIKI-1522

I'd love to get feedback on this. Here's what I say in there:

"
At the place where you want to register for an event:

getObservationManager().addListener(new DocumentSaveEvent("DocumentToWatch"), listener);

At the place where you want to receive for an event:

public class MyListener implements EventListener
{
        public void onEvent(Event event, Object source, Object data)
        {
            ...
        }
 }

At the place where you want to fire notifications to event listeners:

getObservationManager().notify(new DocumentSaveEvent("SomeDocument"), "some source", "some data");

It's easily extensible and you can create all sorts of events as it's fully generic.
"

Have a look at the implementation. You'll see I have even introduced the notion of EventFilter so that we can have regex when registering interest for an event.

Thanks
-Vincent