Steve Hays drew my attention to an article by N.T. Wright arguing for the virgin birth. Wright makes some good points, but some of the best arguments for the virgin birth and some of the best responses to objections to it aren't addressed. Here's an article I wrote earlier this year that outlines some of the evidence for the virgin birth and links a lengthy book review I wrote on the subject.
Among other points I make in my article linked above, it should be noted that the virgin birth receives indirect support in the citation of Luke's gospel as scripture in 1 Timothy 5:18. Though John doesn't refer to the virgin birth in his writings, what he does write is consistent with the doctrine, and it's affirmed early and widely among the disciples of John and the churches he most influenced. The virgin birth was affirmed by Polycarp and other contemporaries of the apostles, as I document in my material linked above. Christian sources in the second century refer to the virgin birth as a core belief of Christianity that's accepted across the Christian world. I've also argued that early and widespread belief in the virgin birth is the best explanation for why the premarital timing of Mary's pregnancy wasn't more controversial in early Christianity. We have far more evidence for the virgin birth than the material found in the opening chapters of Matthew and Luke.