I'm writing a silverlight app for facebook and I came across a couple of blogs that were extremely helpful on the subject of duplex services in silverlight.
then this one helped know what to put in the web.config/app.config to setup the service host
Possibly more to come as I figure out how to implement some of this.
--UPDATE: i tried this out and it just locks up. Never got it working. Plus it's polling so it's probably not scalable so useless for what I'm doing.