Yes, I think this is a good idea. Actually, we do it here all the time, and we eventually have to do it because of a simple reason:
We use Remoting to access server functionality. So the Remote Objects on the server need to implement the interfaces and the client code has to have access to the interfaces to use the remote objects.
In general, I think you are more loosely coupled when you put the interfaces in a separate project, so just go along and do it. It isn't really a problem to have 2 assemblies, is it?
ADDITION:
Just crossed my mind: By putting the interfaces in a separate assembly, you additionally get the benefit of being able to reuse the interfaces if a few of them are general enough.
