Here is an interesting story about what can happen when personhood becomes the standard of rights bearing rather than moral value determined by our human nature. Wesley Smith writes a great post about a group seeking to legally identify a chimpanzee as a person.
The term is so meaningless that animals can be argued to be persons while unborn humans are excluded. This is not the argument of one fringe group. Singer utilitarians endorse the rights of animals over humans under certain conditions dependent upon the utility of the human in question.
HT: Second Hand Smoke