Hi messanjah.
One possible approach to your question would be to attempt to define the point at which an embryo becomes "human", rather than "potential human". IMO, I think the line is a little hazy, but an operational definition might be the point when the neocortex begins functioning. After all, it's our brains that make us human (at least as compared to other critters). Of course, this definition probably won't satisfy either side of the debate, but it's at least a place to start the discussion on a
rational vice
emotional base.
BTW: For reference, my position on the subject is nearly 100% congruent with Mr. H's.