In Dev10 Beta 1, the Silverlight-enabled WCF Service template is the version used for Silverlight 3 beta.  This uses a custom binding to enable binary encoding, and is incompatible with Silverlight 2 applications.  If you attempt to add a reference to it, you’ll see errors like the following...

In Silverlight 3, we’ve introduced support for binary message encoding for WCF services, which should give a performance boost. Silverlight 2, as you may recall, only supports basicHttpBinding. The new binding is the default for Silverlight-enabled WCF services created with the Silverlight 3 Tools. ...