You would need to use some form of ajax push to get an event from the business logic to the view layer
how bout @RaiseEvent with @Observer with a4j:push?
how bout the scenario where a user logs out and you @RaiseEvent that a user has logged out in your backing bean and then use @Observer to a4j:push to all other current users of the app that user foo has logged out (as a rich:modalPanel).
can you do that in a Seam app? most likely would involve Application scoped component(s).
typically we concentrate on one session/client at a time (unless it's a workflow/BPM scenario).
anybody tried this before with Seam?
I can't say for a4j, but it works great with ICEfaces.