Woo Suk Hwang and his team have led the world in theraputic cloning and stem cell research. They were the first to create stem cell lines from a cloned human embryo. They recently announced that they plan to create hundreds of new stem cell lines every year.
Apparantly there have also been rumors that the eggs for these cloned embryos came from one of Hwang's junior team members, which is always an ethics violation because it smacks of coersion. The team member may have also received illegal payments. Now a team from the University of Pittsburgh is pulling out of a collaboration with Hwang's team because they are convinced these accusations are true. I think it would take strong evidence of serious wrongdoing to get people to walk away from this much money.
This is the WaPo article, and this is the bioethics.net take on it. Also see the comment at bioethics.net on the Korean bioethics community, which has long distrusted Hwang.