From the "What-Could-Possibly-Go-Wrong?" department - The National Institute Of Health (NIH) is expecting to "to lift a moratorium on funding for controversial experiments that add human stem cells to animal embryos, creating an organism that is part animal, part human."
Now, it should be pointed out that they are not planning to engineer a race of "monkey-men" or "pig-men". Not anytime soon, anyway. The planned uses are many and varied, such as growing human-usable tissue and even organs in animals hosts, and for other research.
However - and you are seeing me at my most cynical here - in my mind, science is often the grown-up equivalent of a little boy poking something with a stick. He will continue to poke until he gets an answer, or gets into trouble.
I tend to believe that we do not understand some things as well as we think we do, and some of this research will eventually lead to...well, we don't really know what. Perhaps "something wonderful" to quote Arthur C. Clarke's David Bowman character in 2010: The Year We make Contact .