Update a page without using a timer or manual postback?

We have a desktop based application that listens to a Windows service via a remoting callback class.  The service can raise an event at which point all listeners will update their display.  
I'm trying to determine if this same kind of behavior can be achieved in ASP.NET using AJAX or even Silverlight.
In a test page, I set up an UpdatePanel and a Timer.  This works smoothly, but I want to avoid making unnecessary calls to the server every 5 seconds.
Is there any way to update a page automagically?
Thanks
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