OOP (including things like generics and interfaces) and the SOA (Service oriented architecture) world don't almost match up entirely.
This might be one of those cases. The WCF system is geared towards sending messages between client and server, and thus, it's always about real and concrete implementations; WCF messaging doesn't handle interface, and as far as I've experienced, it doesn't handle generics all that well either.
This is especially true if you consider the interoperable nature of WCF - it's after all NOT "just" a .NET technology, but an interoperable SOAP (and REST) stack that could be talking to other systems that have no clue about generics (yes, those still exist).
Anything you want to send back and forth between client and server as a SOAP message has to be in a form that can be expressed with a XML schema (your DataContract) - I don't see how you want to model a generic MyType<T>
in XSD - it's just not designed to do that.
So as much as I like generics, too - I don't think they're a good fit to a service-oriented scenario. So I guess you'll have to redesign your services a bit to take that into account.
Marc