This is a known bug in both DevForce 2010 and 2012 which we haven't fixed yet. This isn't actually a known type issue, but a problem with the type mapping needed to pass objects of generic types between tiers. The problem will occur when the client and server assemblies have different names and DevForce hasn't yet "seen" the type and assembly.
The workaround is to use the same assembly name for both the SL and desktop projects. Another option, although I'm not certain it works in all cases, is to rearrange your code somehow so that one or more entity types in this new assembly have been queried before running this projection query.