Scientists are aware of this pitfall and are looking for ways to control the ES cells. For example:

"There is a risk with using immortalized cells — they may be more likely to develop into tumour cells. A way around this problem is exemplified by the work with immortalized liver cells, which were generated with a 'cell-suicide' gene that could be activated by administering a drug to the rats. Strategies of this kind could allow cell numbers to be controlled and immortalized cells to become a clinically useful technology."

From:
Nature 406, 361-364 (27 July 2000) | doi:10.1038/35019186
Stem cells — hype and hope

Ron McKay

Studies of stem cells will help in understanding the development and function of organs in mammals. They may also offer a way of treating diseases ranging from liver failure to Parkinson's disease.

http://www.nature.com/nature/journal.../406361a0.html