I am a very strong supporter of stem cell research, in whatever form it takes, embryonic or adult cells. I do not believe there are any serious ethical issues involved in using these for research. My explanation of my position is a bit long so you can continue after the break.
The Basics
A blastocyte is a group of about 100 cells that, after natural conception, will implant in the uterus. When a scientist is attempting to generate a culture of stem-cells to use for research, they take a fertilized egg and allow the zygote to grow into a blastocyte. The resulting ball of cells has an inner and outer layer. The outer layer is removed and the inner layer is cultured, often for years, and is what is used for research and potentially for treatment of diseases. The stem-cells that are acquired through this process can basically be grown into any type of cell in the body. They do not have characteristics that identify them as a specific body part. If they are cultured in nutrients they will continue to be neutral stem-cells unless given some stimulus to form a more specific organ.
Potential Life
A blastocyte is what is formed before an embryo implants into the uterus and has the opportunity to form a fetus. To define it as "potential human life" you must define what potential it has. Does it have the capability to grow into a fetus? Without a uterus, the answer is no. It cannot grow into a human being without being implanted. If it were to divide millions of times, it would still only generate more stem-cells, it would never grow into a human. To make this as simple as possible: for something to be potential human life, it must be able to grow into human life. A blastocyte, once implanted into a uterus is potential human life, one that is not implanted, can never be human life, therefore, it has no potential.