After some discussion, Mike Flasko (Program Manager, ADO.NET Data Services) has clarified the relationship between the Silverlight 2 Beta 2 Data Services Library and the SP1 of VS2008/.NET 3.5. Specifically he says:
While the Silverlight Beta 2 data services library may work in some scenarios against a .NET Framework 3.5 SP1 RTM server, you should NOT count on the two being fully compatible with each other.
For me, I am going to wait to roll up to SP1 RTM until Silverlight 2 RTM’s. Caveat emptor. Follow the above link to read his full discussion.