You may recall that accumulated mutations leading to cancer within embryonic stem cell lines were identified as a potentially serious problem - if lines became unusable at a fast rate, it would greatly impede progress. The Australian reports that the problem is on the way to being solved: "scientists have overcome a hurdle in the evolution of embryonic stem cell technology by devising a means of weeding out cells that are potentially cancerous. The [research] represents a breakthrough for the technology, whose critics have used this instability to argue stem cell research be halted. ... the research team also believes the discovery may allow abnormal cells to be identified and then purged, maintaining the health of stem cell cultures."
Link: http://www.theaustralian.news.com.au/common/story_page/0,5744,18283615%255E23289,00.html