In his book, The Spirit of the Liturgy, Cardinal Joseph Ratzinger commented:
"The claim used to be made that December 25 developed in opposition to the Mithras myth, or as a Christian response to the cult of the unconquered sun promoted by Roman emperors in the third century in their efforts to establish a new imperial religion. However, these old theories can no longer be sustained" (pp., 107-108).
Joseph Ratzinger (Pope Emeritus Benedict) is certainly no slouch as a scholar, so you had better believe there is solid data behind that statement. Reading down in the comments on Akin's post I came across this link to Dr. Taylor Marshall's article, with a stunning chronology offering an explanation for why Christians began celebrating Jesus' birth on December 25th. Please read it to check out the details. Here is the rough outline: "We can discover that Christ was born in late December by observing first the time of year in which Saint Luke describes Zacharias [a priest, and father of John the Baptist] serving in the temple (Lk.1). This provides us with the approximate conception date of John the Baptist. From there we can follow the chronology that Saint Luke gives and that lands us right smack at the end of December."
Very interesting. I look forward to reading more.