Usually the line is not drawn around "ability to feel pain", it's drawn around "has a central nervous system", which is used as a rough proxy for "is sentient". Otherwise it would be OK for a vegan to kill and eat a person who has lost the ability to feel pain, but otherwise functions normally.
Cool link! And I heard that argument before in my trolling attempts - usually I answer like this:
A central nervous system is defined as "the complex of nerve tissues that controls the activities of the body". Plants don't have "proper" animal nerve tissues, but they do have a complex system that controls activities of the plant, by sending mRNA/proteins through the phloem to the parts of the plant that need regulation (a lot of it is local regulation but psssst, this kills the trolling)
Nice link! Never heard of that before. But if (non-conscious) IBM Watson has a quotient of +12, and humans have one of +13, what good is it for?
Edit: came along this interesting link about the measure of sentience: http://en.wikipedia.org/wiki/Sentience_quotient