Recent philosophy student Ned Resnikoff has a nice article in Salon about what makes something count as being a person and having rights, in which I present a hedonic utilitarian view of what constitutes a person. Joshua Knobe, an excellent philosopher whom I've had the pleasure of meeting, articulates an opposing position (which I don't know if he accepts).
I think the problem he points out at the end of the article -- that some entities might exhibit all the outward behavior associated with personhood without being persons -- isn't entirely intractable. We've got some kind of handle on which regions of the brain are responsible for pain experience, enough that we can cut them out as a therapy for crippling chronic pain problems. If we can find these parts or something neurologically wired up like them in another creature, that gives us some reason to think it can experience pain. Of course, this isn't going to be much help if we ever encounter aliens and want to figure out whether they're conscious. Hopefully consciousness research will have advanced further by then. But for creatures not too far from us, it gives us some handle on whether they can experience pain, which on the utilitarian view I offer is the important thing.