William Hurlbut, "an earnest young member of the [President's Council on Bioethics] conservative wing", has come up with a novel, if not creepy solution: harvest the cells before they become an embryo.
As Hurlbut puts it in his presentation paper, "Incompletely constituted or severed from the whole, subsystems with partial trajectories of development may temporarily proceed forward with a certain biological momentum." In other words, the parts of an embryo—or the parts that normally would become an embryo—might produce stem cells, even if, to avoid the moral problem, we kept these parts incomplete or severed.
Basically, he wants to turn off a gene that tells the cells "you become an embryo". The cells continue to develop, eventually producing the desired stem cells, but never become a defined organism. Of course, not just stem cells form. Other things, called "organ primordia" also form. That's limb and organ "primordia", "a ball of tissue, grown inside some poor creature, full of bits and pieces of what would have been a body." Definitely high on the shiver scale.
I think it's a brilliant solution, if it works. I have little confidence that it will ever be a long-term, widely-used technique. While conservatives have a high tolerance for activities that aren't covered by their beliefs, they also have squeamish stomachs. All it will take is someone posting picture of "organ primordia" on a high-profile religious website and opinions will turn in a hurry.