Not talking about eating and putting on weight, I am talking about mental/social developement and such. Of course you can take a pup away from it's mom at 6 weeks, but does that mean you should?
http://www.cesarsway.com/tips/puppytips/puppy-development-stage-3
http://bullmastiffinfo.tripod.com/when.html
http://www.dogguide.net/blog/2008/10/ask-the-dog-guide-what-age-should-a-puppy-go-home/
http://answers.yahoo.com/question/index?qid=20080825101909AASgOQt
http://www.k9web.com/dog-faqs/new-puppy.html#separate