We are still working on this issue (it seems quite complicated), in the meantime here is a workaround.
Create a Mock Class of the problematic interface.
public class MockIFoo : IFoo
public double[,] Foo()
throw new Exception("The method or operation is not implemented.");
Now in your tests use MockIFoo instead of IFoo.
// Mock our interface - use Concrete Class instead
MockObject mock = MockManager.MockObject(typeof(MockIFoo));
mock.ExpectAndReturn("Foo", new double[5,4]);
// Get our concrete mocked Object
IFoo foo = (IFoo)mock.Object;