I've had dogs all my life, and three kids, and I would never (again, on purpose) have a new dog in the house at the same time as a baby.
Not for social and jealousy reasons (most of those are trivially dealt with). Not for allergy reasons (contrary to what another answerer said, there are plenty of studies showing that exposure to pets while young strongly reduce allergies: http://www.cnn.com/2006/HEALTH/conditions/04/05/cohen.allergies/index.html for instance)
Why not then? Because it's a lot of work. And you will be tired. Babies take a lot of time, puppies take a lot of time, and honestly, you won't have any to spare. Most new parents are operating on half a gas tank anyway for the first few months, and you won't feel like taking a puppy out to pee at 3 am when the baby was up at 2 for a night feed.
If you do this, I would get a puppy when the baby is older, and in a routine, or an oolder already trained dog (getting a puppy before the baby comes might sound like it'd work, but remember that many breeds go into spastic i-don't-listen-anymore teenagerhood around 8~14 months old, and you don't want to time that to be when the baby is newborn anymore than you'd want a new puppy.