It looks like they actually hit it.
Stem Cells have been discovered inside of embryonic hearts that may actually be the basis for all three kinds of heart tissue (the smooth muscle, the striated muscle, and the endothelial cells. (Cells that line the heart and all of the blood vessels). These cells contain a protien called Islet-1 that has been found before in embryonic mouse hearts, and recently researchers at Massachusetts General Hospital found them in human embryonic hearts. The initial hope is not that Islet-1 Stem Cells can be manipulated in to cures for various heart problems – you need a lot of Stem Cells to affect any portion of the body, more than the normal human usually has available – but that heart tissue can be grown as a template, and used to study the effects of various heart drugs.
An unexpected bonus is that these stem cells seem to be more prevalent in the areas of the heart that trigger Congenital Heart Defects. Obviously much more study is needed – researchers are years away from even a test of their theory – but this looks promising.