Facebook will soon present us all with the feature that will let us know if we are signed-in on any other location or device. It will also give us detailed information about the location and the nature of the device and the option to sign-off those other devices/places. This is a common feature for GMail and AIM and some other similar services. It is not clear yet when this new feature will be rolled out. So for now we can only wait for the announcement to be made.