John's Baptism and Christian Baptism

On page 162 of The Cambridge History of Christianity: Origins to Constantine, Wayne Meeks says the following:

"Although three of the four canonical gospels recount Jesus' own baptism by John, it is not connected expressly with Christian baptism until the early second century, when Ignatius says that Jesus was baptised 'in order to purify the water by his own submission [or suffering]' (Ign. Eph. 18.2)."

The reason that this passage stood out to me is that I have wondered about the relationship between the baptisms conducted by John the Baptist and those conducted by early Christianity.  As Meeks goes on to document, early Christianity viewed baptism as a time when the initiate was united with Christ in his death and resurrection, resulting in a new person.  But did it believe that John the Baptist's baptisms had this significance, when they occurred way before Christ's death and resurrection?  This question first entered my mind when I was following debates about whether or not baptism is required for salvation.  People who answer "no" appeal to the thief on the cross as one who was saved without being baptized.  There are a variety of responses to this argument from those who answer "yes", but one that I've heard is that we do not know for sure that the thief was not baptized.  After all, John the Baptist performed baptisms, as did Jesus' disciples.  But did such baptisms result in the death of the old man and the birth of a new one?  And if early Christianity's baptisms were the first to have this significance, then perhaps people before the death and resurrection of Jesus could be saved apart from water baptism.

I can't say that I understand what Ignatius is saying.  Is Ignatius' point that Jesus, by submitting to John's baptism, was purifying the waters of John's baptism such that they could bring forgiveness of sin?  Or is his point that Jesus' passion----which was yet to come----was being retroactively applied to the waters of John's baptism when Jesus was baptized, thereby purifying them?  Or is Ignatius saying that Jesus purified the ritual of baptism itself----not so much John's baptism?  In this scenario, baptism before Jesus' death accomplished something, but it was imperfect.  Jesus, by his death and resurrection, made baptism into a ritual of death and rebirth, meaning that Christian baptism was an improvement upon John's baptism.

In the Gospels and Acts, could John's baptism have anything to do with the death and resurrection of Jesus, or Jesus, period?  The synoptics present baptism as a ritual that brings forgiveness, but there is no reference to being united with Christ in his death and resurrection.  In the Gospel of John, John the Baptist appears to be aware that Jesus will die, for he calls Jesus the lamb of God who takes away the sin of the world.  I realize that there is debate about whether John's Gospel is anti-ritual, and that the Gospel of John does not mention the significance of John's baptism, but I can understand why one could argue that John the Baptist in John's Gospel baptizes in light of the coming death of Christ.

In Acts 19, Paul appears to re-baptize people who had only received John the Baptist's baptism.  E.W. Bullinger, however, interprets Acts 19 differently.  He thinks that v 5 is still part of Paul's speech: that Paul is saying that people were baptized by John in the name of Jesus.  This may or may not make sense, but there is a sense in which Jesus was relevant to John's baptism, for John in the synoptic Gospels was baptizing people in preparation for the coming of Jesus.  At the same time, if the people being baptized by John knew about the significance of Jesus, why do Acts 18-19 present people who only received John's baptism as essentially clueless about Jesus, which was why they needed to be instructed by Christians?