I would say that's more a question of them being conscious or not.
A human in a persistent vegitative state, for example (lol, Schaivo controversy!), would be human by definition, but brain dead. A mass of living human cells that are being kept alive artificially despite the fact that the parts of the brain that control conscious thought have been severed. A functionally dead human.
A human that has been blown up, for example, would still be human. Just physically dead.
Something that is able to think like us isn't necessarily us. What interesting school of philosophy do you subscribe to with this nonsensical (to me) talk of severely retarded people not being worthy of the title "human"?