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:

http://betaforums.silverlight.net/forums/t/126267.aspx

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).

http://msdn.microsoft.com/en-us/magazine/cc794260.aspx

I use this for my development server / Cassini:

ClientAccessPolicy.xml

<?xml version="1.0" encoding="utf-8" ?>
<access-policy>
  <cross-domain-access>
    <policy>
      <allow-from http-request-headers="*">
        <domain uri="*"/>
      </allow-from>
      <grant-to>
        <resource include-subpaths="true" path="/"/>
      </grant-to>
    </policy>
  </cross-domain-access>
</access-policy>

You are serializing an Enum and failed to decorate the Enum Values with the EnumMemberAttribute.

http://msdn.microsoft.com/en-us/library/system.runtime.serialization.enummemberattribute.aspx

Still stuck? Turn on tracing and your stack trace will show the true error. (THIS WORKS!!)

http://blogs.msdn.com/madhuponduru/archive/2006/05/18/601458.aspx

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?

http://projects.nikhilk.net/WebDevHelper

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.