[MarcusTulliusCicero]

The current state of stem cell research is analogous to one of us going into a nuclear power plant and flipping random switches to see what happens. Just to get a cell to grow outside of it's normal environment, you introduce changes. Further manipulation of the stem cell to mutate the DNA can also have other unforeseen consequences. By the time you are done and want to reintroduce it back into the host, you clearly have no idea all of the things you have actually done to it. And chances are actually very high that you are introducing a source of future cancer stem cells into the body.

[LibFreeOrDie]

At least one recent study suggests that the longer the stem cells hang around undifferentiated in vitro, the greater the chance of them turning cancerous in vivo.

Unfortunately, I don't have a link to the article I read. I believe it applied to both embryonic and non-embryonic stem cells used in animal studies.