Here is a sticky from the Breeder Section that breaks down the developmental stages by week for pups.
http://www.yorkietalk.com/forums/showthread.php?t=31003
Again, in many cases, pups brought home that age are fine, but in some, they are not. There have been studies indicating it is best to wait until they are older in addition to the advice of 12 weeks being the standard answer you will get from most long time, respected breeders. Doesn't that mean something? Is it really worth taking the risk?