There were a couple of changes related to the way Rhino Mocks handles out and ref parameters in versions 3.6 and 3.6.1, so I would change the test approach to something like this:
var userid = user.Identifier;
UserRepository.Stub(x => x
.GetUser(Arg.Is(userid), Arg.Is(long.MinValue), out myVar)
.OutRef(valueForMyVar)
.Return(user);
And if you really wanna make sure you're calling the method, you can use UserRepository.AssertWasCalled at the end of the test...