I've been thinking about Anon's comment below some more. (Thanks Anon! :) ) and I'm thinking the question doesn't really even come down to "When does human life begin" but rather, "What is a person?"
How do we decide what a person is? Well, we can decide based on the law. As in "The law defines x as a person, but y as not a person, so it is ok to kill/enslave/exploit y, but not x"
The problem is, when we allow the law to make some humans people, and some not-people, it's pretty arbitrary. (See slavery. And women under the Taliban). So there has to be a universal definition of "Person."
Christianity takes, I think, the broadest possible definition: All human beings. Because God gave them souls. Which is why the Church is against abortion.
Our current society seems to take the definition as "Anyone born. Unless they have a terminal disease, are in a coma, want to die, or are in this country illegally."
Personally, I find the latter definition, and the way that it's ever expanding, really creepy.
But from a non-religious perspective, what makes a person?
Is it Usefullness to society? Not really, since it's wrong to murder crazy old homeless guys on the corner. And we don't go around killing people simply because they use more than they produce.
Is it potential years to live? No, because if a drunk driver kills someone, it doesn't matter if the victim is 16 or 95. He's still killed a PERSON. There don't seem to be degrees of personhood. It's an all or nothing proposition.
Is it memories? No. Because Amnesiacs and Alzheimers patients are still people.
So how DO we define a person? And what's our reason for choosing that specific definition? (And if we define person more narrowly than others, are we doing it for selfish reasons? Like the factory owner who thinks it's ok to pay illegals an unjust wage, or the plantation owner who lives a life of luxury because he can exploit the slaves?)