The issue with using embryological stem cells (ESCs) is that there isn't a definitive time when "life" is considered to have begun. Few people argue that a 40 week fetus is a living, viable baby. The contested points at which people consider a zygote/embryo/fetus living are: preconception, fertilization, implantation, 8th week, quickening, 20th week, 25th week, and birth. Personally, I'm not sure where that line is, but I definitely don't believe it is preconception, fertilization, or any other time before implantation, and I don't believe that the threshold for life is after development of the functional nervous and cardiovascular systems. New genetic material, as produced by the mixing of the two parents' DNA, is not a definitive marking for new life, in my opinion, and the embryos that are used in ESCR are blueprints and raw materials for human beings, not necessarily human beings themselves. I can understand how people would believe that life begins at almost all of these points, but I don't buy the "potential to become a human" argument, nor do I buy the "fetus isn't breathing, and therefor not living" argument.
I do not have any qualms about embryonic stem cell research. The benefits of that research extend beyond those we can get from adult stem cell research, as we are just finding ASCs that are pluripotent (capable of differentiating into any type of cell), and considering the fact that those embryos would likely never be implanted and thus never become humans, I don't see anything wrong with using them. Hopefully one day, we'll be able to use ASCs exclusively, but until that day, I have no issues with using embryos.
http://brainblogger.com/2009/05/10/medical-controversy-when-does-life-begin/