I found that different interfaces have different behaviour on different device (router, switch) when no cable is attached on Packet Tracer 6.0.1.0011.
Router's serial : down down
Router's ethernet: up down
Switch's : down down
What is that so special for ethernet? For serial, I was informed that the status will be up down if a cable is attached but has some mismatch for L2 configuration.
Note: I don't know if the behaviour is same if a cable is attached on the interface but the other end of the cable is not attached to any device.