Debate on the ethics of research using cells from human embryos still roars today, and most of it seems to be either missing or deliberately avoiding a crucial point: There are embryos that are being destroyed which could be used for research. There is no question of preserving sacred human life in this situation; indeed, these embryos ARE NOT going to live on. This is something that happens regardless of whether their cells are used for research. They will not be used for reproduction and thus do not survive.
The lives of humans not yet past their embryotic stage are not more sacred than the lives of humans with diseases or problems that could be addressed with embryotic stem cell research. The prevailing popular argument against research on embryotic stem cells - that they violate the sanctity of human life - is a self-defeating point.
It is almost indisputable that stem cell research could save and improve many lives. Note that stem cell treatments can do more than just save lives; they can make better the lives of many who have horrible difficulty facing life each day due to their ailments. 
Either decision on the issue of whether to continue research using specifically embryotic stem cells, is an effective denial of life to one human or another. Allowing research to continue prevents the embryos used from maturing into humans. But disallowing use of embryotic cell gathering and research to continue prevents the plethora of prospective medical treatments and cures first from coming into existence and second from being deliverable once known; this will directly deny survival to any who could have used these stem cell cures to survive, especially when nothing else works or is available for their particular illnesses. BOTH subjects are innocent. Human embryos are no more sacred than humans who have lived their lives partially, and simply need help to live the rest of it.
Avoiding an avenue for solving a problem is tantamount to making that problem worse.
Adult stem cells are not a valid replacement for embryonic stem cells. Adult stem cells are "rare, often difficult to identify, of unknown origin," "can divide few or many times (up to 200 or more)... in culture, but not indefinitely," and are far less flexible: "most only give rise to their tissue of origin, but some may be able to give rise to different cell types within the larger groupings of endoderm, mesoderm, and ectoderm." In short, Adult stem cells are harder to get, produce more limited samples, and aren't as versatile as Embryonic Stem Cells. That is not to say that there is no use for Adult Stem cells; the simple fact is that they aren't valid as a _complete_and_sufficient_alternative_.
It is possible, that treatments for certain diseases and ailments may be found without relying on embryotic stem cells. But even with alternatives possible, there will still be those falling victim to their respective troubles who would have been saved by the extra boost in research and usability gained from the readily available and versatile embryotic cells.
 "Stem Cell Research: Status, Prospects, Prerequisites." http://www.embo.org/scisoc/stem_cell_11_dec.pdf
 "Stem Cells and the Future of Regenerative Medicine." Board on Neuroscience and Behavioral Health, Institute of Medicine. http://books.nap.edu/openbook.php?record_id=10195&page=26
 "The Forgotten Embryo." Carl T. Hall, San Francisco Chronicle. http://sfgate.com/cgi-bin/article.cgi?file=/c/a/2001/08/20/MN58092.DTL
 "Human embryos and fertility clinics." Ontario Consultants on Religious Tolerance. http://www.religioustolerance.org/abo_inco.htm