So here is the skinny: Your WCF service blew up and that was the extent of your error.
Solution: Figure out why your WCF service blew up.
Some possible solutions:
You are returning too much data and your service config is causing the problem:
Increase your array length or your data length.
You are querying a WCF service cross domain (yes if your wcf service is hosted in Cassini you need to have a correct cross domain policy).
I use this for my development server / Cassini:
<?xml version="1.0" encoding="utf-8" ?>
<resource include-subpaths="true" path="/"/>
You are serializing an Enum and failed to decorate the Enum Values with the EnumMemberAttribute.
Still stuck? Turn on tracing and your stack trace will show the true error. (THIS WORKS!!)
Test your service with the WcfTestClient that comes with visual studio.
This will allow you to call the WCF service with a simple little tool. The GUI allows you to double click on a WCF method and provide the parameters for the method. The app calls the WCF service and reports the stack trace.
The GUI app is found here on 64 bit computers:
C:\Program Files (x86)\Microsoft Visual Studio 9.0\Common7\IDE\WcfTestClient.exe
and here on 32 bit machines:
C:\Program Files\Microsoft Visual Studio 9.0\Common7\IDE\WcfTestClient.exe
What is being sent to my WCF service or what is being sent or received by my Silverlight application?
Note: This tool is VERY good for checking security on your Silverlight application. I recently heard a company was hacked because they transmitted state information via XML to a server. The user used an app like this or Fiddler to see the communication between the server and the browser and modified the response. They stole a lot of $ by doing this. Be WARNED! Users are getting a lot smarter, do not assume your XML traffic is safe.