A Cambridge University group have published in Nature that human ESC are much more stable tha originally thought:

http://news.bbc.co.uk/1/hi/health/4596833.stm

Unlike similar cells from mice, their epigenetic status hardly altered at all while they were grown in culture.

Researcher Professor Roger Pederson said it was vital that human stem cells were shown to be stable before research could forge ahead.

"We know from mouse stem cells that these kind of genes are altered in the course of mouse embryo development.

"The fact that human stem cells are so stable is good news."

........He said it was not known exactly what effect using unstable stem cells would have, but it was possible that the cells would either over-grow, or not grow at all, rendering treatments useless.