I agree jabber, a baby certainly hasn't reached the age of accountability (which varies by faith), nor does the baby know what sin is - it's soul might, but as human, no. They don't have the capacity or understanding to place their faith in Christ, which is what baptism is all about, a declaration of faith. [Ref Acts 2:38]
In my belief, the parents bare the sins of the child up until he/she is able to know right from wrong, again the age of accountability.
The scriptures tell it like it is: And [Jesus] said, Verily I say unto you, Except ye be converted, and become as little children, ye shall not enter into the kingdom of heaven. (Matt 18:3) To Jesus, (the touchstone of baptism) children were of the purist nature, unmarked by sin.

celtic, I'm saying this, not because I disagree with baptisms of babies, but because I honestly believe it not to be Biblical. Without having taking on the robe (faith) of the One for which we are being baptized, baptism is just a ritual (maybe even a 'feel good' ritual for the parents). I mean no disrespect in saying that, but I know of no Biblical support for the practice... but I will certainly accept any proof given. We're here to learn, not oppose.