There is a new CTP out for WCF Data Services (v3), which features the long awaited support for properties on derived types (yay!)
You can get it from here.

The following proposed OData version 3.0 functionality is also supported by WCF Data Services in this release:

• Multi-Valued Data Types
  OData now enables you to define multi-valued types in the data model of your data service. These multi-valued types contain unordered collections of primitive or complex types.
• Named Resource Streams
  OData now enables you to define named resource streams for a given entity. This gives you the ability to have more than one binary data stream associated with a given entity. The .NET Framework client library now enables you to access named resource streams.
• PATCH Requests
  A new PATCH method has been added to the HTTP standard. OData now supports this new HTTP method. WCF Data Services handles PATCH requests in the same way that it handles MERGE requests. The .NET Framework client library now enables you to request that updates be sent to the data service by using a PATCH request.
• Prefer Header Support
  OData now supports the ability for clients to request whether or not a payload is returned in response to a POST, PUT, MERGE, or PATCH request. This client preference is indicated in the request by the value of the Prefer header. When using the .NET Framework client, this preference is managed by the DataServiceContext.

The following additional functionality is also provided by WCF Data Services in this release:

• Properties on Derived Types
  You can now explicitly create relationships between derived types. Previously, you could only do this on the base type.
• Expanded Support for Feed Customization
  Feed customization now supports mapping to the atom:link and atom:category elements and mapping multi-valued properties. You can now also define conditional criteria when mapping to the attributes of specific elements in the returned feed.
• Support for Entity Sets with Different Base URIs
  The OData protocol allows for a data service to expose entity sets as collections that have different base URIs. Previously, the .NET Framework client assumed that all entity sets shared the same base URI defined in the DataServiceContext. Now, you can define a delegate that is used by the context to resolve URIs for entity sets that do not share a base URI.
• Including Relationship Links in the Response
  The OData protocol defines a method for addressing relationships between entities by using the $links operator in a URI. WCF Data Services now enables you to request that the data service include these links in entry elements in the response. This behavior is controlled by the IncludeRelationshipLinksInResponse configuration property.
• The .NET Framework client library now uses relationship links when constructing URIs that address related entities, when they are present in the response.

 Glenn Gailey