I have a similar need, and I saw this article about the Comet-esque feature that they've provided for Silverlight 2 over WCF: Silverlight Polling Duplex.
I haven't tried it yet but I'm thinking that the assembly built against the desktop runtime may include the client classes as well, if that's the case then this may be usable outside of Silverlight.
Edit: I checked both assemblies and they both implement the same Bindings and Channels, it looks like the same code just built against the desktop framework; so you should be able to use the "Server" assembly in a desktop application.