I know you are getting a variety of responses, But I would like to applaud you for not forsaking the life long commitment you made to your dog because it might be the easy thing to do. Believe me, I see these poor homeless kids everyday and both your dog and your son will benefit from your commitment.
I am currently working with my heeler in breaking some overly zealous play habits before my child arrives. In regards to techniques, I have many pros recommend getting a short (4 foot) leash and a harness with a ring on the chest for the leash to attach to and have the dog wear the leash when ever guests or the baby are around (basically all the time but nap times and such). This way, when the dog is behaving unrully toward the baby and ignoring your correction, pull the leash towards you and the dog is now facing you. This startle technique has been really effective when my friends have small children visiting, and as a heeler, my dog would really like to herd small children.
Also remember that while you see your dog as part of the family, your dog does not see the baby as part of the family yet. The babbling and noises made by small children is reminecent of the noises prey animals make. Also, remember that your cute yorkie has ancestors that were bred to hunt, not to be cute. The terrier personality traits that get untrained pits in trouble are the same traits your yorkie has . . . they are all terriers and were bred for a common purpose. I am not saying this to scare you, but this is what you are trying to train not to do, so you need to understand that your dog will follow his/her instincts unless you are very persistant about other behaviors (again, I am up against the same thing but with the instinct to chase rather than hunt - either way, instincts are hard to change).
So with all of this in mind, keep your head when correcting your dog - don't correct in an angry or aggressive manner, but in a 100% consistant manner. THe other key thing is that if your dog is loosing privilages that he/she previously had (such as concentrated mommy time, bed time, furniture privelages and so forth) then your dog has more to be upset about. I'm not saying ditch the baby for the dog, but get on the floor and play or snuggle or what ever is normal pre-baby during nap time. make an effor t to not ditch the dog mentally . . . babies are very time consuming and can make us loose track of the rest of our lives, but just like our spouses and older children, pets don't handle being brushed aside for the new kiddo really well. So give your dog some concentrated loving, keep your corrections 100% consistant, get a crate for when you can't monitor the situation completely (on the phone, doing dishes, whatever) and remember that your four legged kid deserves a home just as much as your 2 legged kid, so give the nay-sayers what fore and show them what your dedication to your family can do.
You are setting a wonderful example for your son through your dedication!!
Good Luck!
PS - If you are on the kitsap penninsula, email me and I can give you a a couple of fantastic traineers that will come to your home and work with your situation hands on!