After struggling with WCF RIA Services v1 (Silverlight 4 RTM) to enable insert/update for the OData endpoint for use with Windows Phone 7, I finally figured it out. The current version of WCF RIA Services does NOT support update or LINQ! I think I searched for a couple of hours until I found the single line of information on this in this following Silverlight TV post:

NOTE: The ODATA endpoint has very limited support in V1. There is no Update or LINQ query support in this release.