I'm following up on that case here, still clinging to some hope:

http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=4172128&SiteID=1&mode=1

(This is mostly educational, still under consideration for production use if you should wonder ;) )

Edit 02 jan 2009

Yet another hope stirring:

http://msdn.microsoft.com/en-us/library/bb738631.aspx

Gonna see what happens with the .Cast<>() method if I implement those interfaces on the entities instead of using an expression visitor.

Edit 19 jan 2009

And so he tossed away all the ideas about interfaces for entities as the POCO adapter for L2E arrives :)

http://blogs.msdn.com/jkowalski/archive/2008/09/09/persistence-ignorance-poco-adapter-for-entity-framework-v1.aspx

Thanks to Jimmy Nilsson (http://jimmynilsson.com) for the information.