I found this creepy story at BlogsOfWar - "On a farm about six miles outside this gambling town, Jason Chamberlain looks over a flock of about 50 smelly sheep, many of them possessing partially human livers, hearts, brains and other organs."
Stanford law professor Hank Greely, who chaired the ethics committee, said the board was satisfied that the size and shape of the mouse brain would prevent the human cells from creating any traits of humanity. Just in case, Greely said, the committee recommended closely monitoring the mice's behavior and immediately killing any that display human-like behavior.
As BlogsOfWar puts it, the potential benefits of this technology are enormous but I just don't have much faith in people's ability to conduct this kind of research in a safe and ethical manner(nor do I believe that it is ethical). It doesn't really matter though because the technology and the knowledge are already out there. There's not much to stop it now except legislating ethics - and I doubt that will be done.