US outlawed embryonic stem cell research from all but stem cells from a few existing lines. People lamented that this ban would leave the US behind in an important field of medical research. But I am not so sure that has been the case, as we are now focused on taking mature cells and turning them into cells that can generate new tissues in the right conditions.
In a sense, what the Japanese researchers did here is much more difficult and significant than embryonic stem cell research, because they are taking mature human cells, turning them into stem cells and getting them to turn into a new liver which is then implanted in mice and grown to a larger size.
The best case eventual outcome from this research will be to replace a diseased human's liver with a good one, grown with his or her own liver cells.
And when they can use this technology to re-grow other organs and bodyparts from a person's own tissue, then we'll really have achieved a miracle of modern medicine.