I agree 8 weeks is the youngest a little puppy should leave its mummy - 6 weeks IMHO is to early, the puppy is learning the basic behavioral patterns specific to dogs, its also the time that the mummy dog teaches the pups good manners and disciplines the bad manners.
This might explain it better (taken from
http://www.doberman.org/articles/puppy.htm)
"If a pup has not learned to accept leadership (and discipline) in its early interactions with dogs, its training will be more difficult. Puppies that are removed from the nest too early tend to be nervous, more prone to barking and biting, and less responsive to discipline. Often they are aggressive with other dogs. Generally speaking, a puppy taken away from it's mother and litter mates before seven weeks of age, may not realize its full potential as a dog and companion. To maximize the mental and psychological development of puppies, they must remain in the nest with their mother and litter mates until seven weeks of age."
another great site that takes you through a pups critical periods in development is:
http://www.ccc.govt.nz/animals/CriticalPuppyDevelopment.asp
Even though it quotes until seven weeks of age, i still think you should ask the breeder keeps your puppy with the mother until 8 weeks if this possible?
Hope this heaps - Goodluck with your new baby and cant wait to see some piccies.