How have Christians throughout history regarded Mary, specifically in regard to her perpetual virginity? Is there a major difference in regard to Marian theology that is discernible along the historic Catholic/Protestant divide?

In the modern era, we might answer "yes" to such questions. Nestorianism seems to be making a bit of a return, in the form of modern Protestants and non-denominational Christians actively denying the ancient doctrine of the Church that Mary is the Theotokos, the God-bearer, the Mother of God. But historically, the answer is actually a resounding "no!"

Martin Luther didn't think the doctrine was critical to Christian faith, but maintained that "we should simply hold that (Mary) remained a virgin after the birth of Christ because Scripture does not state or indicate that she later lost her virginity." (c.f. 'That Jesus Christ Was Born a Jew', Luther's Works, 1962, Concordia Publishing House)

Luther was insistent that Jesus was "the real and natural fruit" of Mary's womb, and that she "bore no children besides Him." Additionally, Luther fully agreed with interpretations of Scripture that identified the "brothers and sisters" of Jesus as being more distant relations (e.g. cousins).

Moreover, John Calvin also rejected the notion that the mention of the "brothers and sisters" of Jesus in the Bible were meant to refer to siblings born of Mary. John Wesley believed that Mary "continued a pure and unspotted virgin" after giving birth to Christ. (c.f. 'Letter to a Roman Catholic', 1749) Huldrych Zwingli likewise believed that Mary's virginity remained perpetually intact after Jesus was born. (c.f. 'Eini Predigt von der ewig reinen Magd Maria.', Huldreich Zwinglis sämtliche Werke, 1905)

Diarmaid MacCulloch — a Reformation historian — noted the reason why the affirmation of Mary's virginity was almost wholly universal across all the major strains of Protestantism and its offshoots: by affirming the ongoing virginity of Mary, the Reformers (not unlike the Catholics) ensured themselves "the guarantee of the Incarnation of Christ." Sola Scriptura may have prevented the believe in Mary's virginity from being formally adopted as a doctrine in many (if any) of the churches which emerged out of the Reformation, and I suspect that this may have done a grave disservice to subsequent Protestant theologians where the issue of Mary was concerned, many of whom have since come to deny the doctrine of Mary's perpetual virginity. (c.f. The Reformation: a History, 2003, Penguin Books)

And while it sounds like this should be a minor issue, look at where that denial has led us to: now we see again some Christians stumbling into false teachings like Nestorianism, and uttering denials of Mary as the Mother of God that they have no idea also necessarily imply that Christ was not fully human and fully divine all at once.

The Catholic Church obviously holds the belief that Mary was perpetually virginal, and has held that belief since…well, since the Church was still one denomination, way back when the year took less than four numbers to write out. The formal doctrine of Mary's perpetual virginity was promulgated in about 533 A.D. (at the Council of Constantinople). That's not to say that Catholicism has not been marked by a few disagreements over the issue, though.

Tertulian, for example, believed that the virginity of Mary was not preserved by the act of giving birth. While he evidently felt this was a proper emphasis on the nature and reality of Jesus' human body, it should be noted that in reality, this position actually makes the very idea of a "virgin birth" (an integral part of Christian theology) impossible. Jovinian, who lived over a century later, set himself against Paul (c.f. 1 Corinthians 7) when he denied that virginity/celibacy was indeed a higher state than marriage. Helvidius repeated this commentary shortly thereafter, but was soundly denounced by, among others, Jerome.

And as was so ably noted by Mark Shea, "the Tradition of the Church in union with the biblical text [affirms that] Mary had no other children, a fact so commonly known throughout the early Church that when Jerome attacks Helvidius for suggesting otherwise, nobody makes a peep. In a Church quite capable of tearing itself to pieces over distinctions between homoousious and homoiousious, you hear the sound of crickets in response to Jerome, punctuated with the sound of other Fathers singing hymns to "Mary, Ever-Virgin." The early Church took it for granted and thought Helvidius as credible as Dan Brown."

And indeed, after Helvidius, no credible arguments were presented against the perpetual virginity of Mary until near-modern Protestantism jettisoned the doctrine.

Possible Objection #8: could not the early Church have been wrong? Could the early Church have mis-interpreted those passages from Scripture which make clear reference to the siblings of Jesus?

Response to Objection #8: it's possible, I suppose, that this could be the case. But to believe as much, we would have to believe that over 1700 years of Christian scholarship was wrong about this particular issue, that 1700 years' worth of Christian theologians and scholars had somehow remained ignorant of very commonly cited verses of Scripture, and that in 1700 years nobody had actually thought to read the original-language manuscripts in a proper context.

Which seems a bit…incredible. It makes more sense, I think, to assume that modern theologians who set themselves against the perpetual virginity of Mary have simply gone awry in their thinking, as did the Nestorians, as did Helvidius, and as did Tertulian.