While you can easily use XML web services in your Silverlight applications, working with WCF is much different with the current Silverlight 1.1 September CTP bits. I have done a bit research on the Internet and found a few blogs below that are very helpful.

http://blogs.msdn.com/justinjsmith/archive/2007/07/23/wcf-asp-net-ajax-and-javascript-proxies.aspx
http://www.wintellect.com/cs/blogs/sseely/archive/2007/08/09/accessing-a-wcf-service-from-silverlight.aspx
http://msmvps.com/blogs/luisabreu/archive/2007/06/12/calling-a-wcf-service-from-your-silverlight-application.aspx
http://msmvps.com/blogs/luisabreu/archive/2007/06/13/calling-a-silverlight-wcf-service-part-ii.aspx

All these blogs suggested one option of working with WCF in Silverlight, using Json. After trying several different ways, I was able to put together a working sample and attached it for download. This sample worked on Windows Server 2003 and Windows Server 2008 RC0 with VS.NET 2008 Beta 2 and Silverlight 1.1 September 2007 CTP.

A note: I did try to use WCF without relying on Json. I first created a WCF and host it in IIS. Then I set a web reference to the WCF by pointing to the service.svc url. Behind the scenes the IDE is using the service's wsdl. I then created a Silverlight app, and tried to consume the WCF service in a similar way to what I did with XML web services (using async call). The code didn't return any error, but it did not get any data from the WCF service either. I am trying to figure out if the error was due to data serialization issue, configuration issue, or something else.