ADO.NET Data Services has been renamed to WCF Data Services in Silverlight 4 and .NET 4. The protocol that is talked between Client and Server has also been renamed and is now known as OData Protocoll. You can get a variety of clients and providers that speak “OData” from http://Odata.org.
OData and WCF Data Services is my favorite technology for data transfer for 3-Tier Architectures because it allows me to retrieve, change and update data on the client without requiring me to write additional server side code that handles queries and the like. In addition Silverlight 4 now supports change tracking on the client which makes my T4 Templates I wrote for Silverlight 3 kind of non-any longer needed.
Here is a how do I video that shows you how to expose an existing Database via OData and access it from Silverlight
Video: Expose Database via WCF Data Services
Video: Consume WCF Data Service in Silverlight 4