Both/And

John Shelby Spong, in his 1998 book Why Christianity Must Change or Die (Bishop Spong doesn't pussyfoot around, obviously) discusses (on pp. 75-83) the chronological development of the notion of Jesus as the Christ.

He notes that, as the earliest of the canonical writings, the Epistles of Paul, written between 50 and 60 C.E., not only fail to mention the birth of Jesus, but, in Romans (1:1-5) states that Jesus was designated "Son of God" by the Father only at his resurrection.

Mark, the earliest of the synoptic writers, writing some 20 years later, moves this designation backward in time to the baptism of Jesus (1:9-10), while Matthew, writing 10 to 20 years after Mark, moves even further backward in time to his birth.

Luke, writing 5 to 10 years after Matthew, changes the birth details somewhat, but makes the Annunciation the moment of divine designation.

Finally, John, writing almost 100 years into the Common Era, dispenses with all previous times of designation and proclaims that Jesus shared divine identity from "In the beginning," then states that "the Word became flesh and dwelt amongst us."

As for the noncanonical infancy narratives, many are based on the 2d century Protoevangelium of James, which concentrates on the virginity of Mary, rather than the circumstances of Jesus's birth as well as the Infancy Gospel of Thomas and the Infancy Gospel of James (It's worthwhile to note, I think, that these latter sources are the basis for Anne Rice's recent novel, Christ the Lord. No doubt it'll become a best seller, given the current temper of the times...
Boreas

A quick note, not by way of argument or disagreement, but clarification.

This idea is not original with Spong, nor really all that challenging. Fr. Raymond Brown acknowledges as the same exegetical understanding in his magisterial (and, by modern scriptural studies standards, quite safe and conservative) Birth of the Messiah (a book I like even as I might disagree with it; but then, I disagree with Bultmann's "demythologization," which is basically Spong's shtick).

Mark is dated to around 70 C.E., and contains no mention of a nativity at all. Neither does john, which clocks in between 100 and 120 C.E.

The only story from the life of Jesus of Nazareth in Paul's letters is the reference to the eucharisto in Corinthians. Otherwise, Paul preaches "Christ crucified," which for Paul clearly is what made Jesus of Nazareth "Messiah" (which means the "last supper" for Paul is a memorial meal; the metaphysics of it came much later. Although it raises the interesting question, again: what do the words of institution in Corinthians mean?)

Matthew and Luke both have widely divergent nativity stories, which cannot be reconciled. In Matthew, Mary and Joseph live in Bethlehem, and the wise men come when Jesus is 2 or younger (because that's the age of the innocents slaughtered by Herod). The family flees to Egypt (recaptiulating Israel's history; in with Joseph, out with Moses), and comes back to settle in Nazareth.

In Luke, they live in Nazareth and go to Bethlehem for the census (for which there is no other record than in Luke's tale).

By John's gospel, metaphysics has taken over, and Jesus not only predates Creation, but is the shaping force of Creation (the logos of Greek philosophy is thus reconciled with the Creator of Hebraic belief). Birth is irrelevant to the Word, which exists, with God, prior to the beginning.

What does this mean?

Well, that the gospels are not history texts in the way the Greeks understood them, or even as we, after them, do. They are, in seminary language, confessional documents. They confess a belief in the God of Abraham and Jesus, based on an experience that the communities which created these documents felt could best be expressed in this way. I learned all this in seminary, and yet I still sing "O Come, O Come Emmanuel" every December, still read Luke 2 in the King James Version every Christmas Eve, still believe in the songs of Mary and Zechariah and Simeon and the angels, and even in the dreams of Joseph. I know what it all means, and yet is still means something to me. I can exegete the Magnificat and the gifts of the Magi and even the journeys of the holy family and the visitation of shepherds.

But they still have their confessional meaning, too.