While working with RIA domain services and Silverlight 3 beta, I ran across a strange problem.  I would create the domain service based on my entity framework model, and the code generated was causing an error.  I verified that the entity framework ran on its own, but it took me a while to figure out why I was getting an error similar to the following:

Invalid member projection specification : Projection path 'MyTableId' is 
invalid for member 'MyNamespace.Web.MyTable.MyTable' 

This error message is not very friendly.  The reason why this happened is because inside the EF designer, I renamed the primary key in MyTable (we'll say from MyTableId to SomethingId).  EF has no problems with renaming the primary key field, and compiles without any problem on its own.  Unfortunately when you try to create a domain service, if you add the EF object along with other objects that associate to it (via the primary key), it fails to compile and gives the error message above.  Whatever magic occurs behind the scenes when RIA generates the client metadata, it does not like the primary key to be different.  The only workaround I know of is to rename it back to whatever it was.  Unfortunately, if you are trying to use RIA Domain services with an existing EF model that has changes to the primary key names, you may have to play with it to get it to work.