> For instance, could a person with Down's Syndrome be denied protection from murder?

Since nobody seems to have given a straight answer to this, I will. Someone with Down's Syndrome easily counts as an intelligent being, and is not even close to any borderline.

I suppose your question assumes that, since intelligence comes in degrees, any value based on it must also come in degrees. But instead, most people who see personhood this way believe in some sort of threshold above which everyone deserves equal protection.