I've heard plenty of sermons, read plenty of books, and participated in plenty of discussions that include, in one form or another, the statement that Mary, the mother of Jesus, was a sinner just like us. I've even heard the phrase "dirty, rotten sinner" used. However, there is something amiss here.
If Jesus is God (something that I no longer question; my opinion changed when I met Him), then we must consider the dwelling places of God in the Old Testament as we seek to understand Mary. This also has ramifications for our own existence, as we are dwellings of the Holy Spirit: the symbols of the Tabernacle and the Temple apply to us -- in other words, Leviticus becomes an eminently practical book in the Christian life.
What was the character of God's dwelling places? Holiness. Absolute purity on pain of death or exile, whichever comes first.
So, if Mary's womb is the Holy of Holies, where the Word resides, what does that make the rest of her? The Temple of the Lord.
Will God change His mind about holiness as He takes up residence here? As He takes human nature from her? As His human existence becomes the new Temple that shall be destroyed, yet three days later raised up?
Now, this doesn't mean that the latterly developed "Immaculate Conception of Mary" is necessary, from a Biblical/symbolic standpoint. One would think that after the promulgation of the Protoevangelium of James around the year AD 145, that doctrine would make an appearance: but no dice. It is, rather, a logical extension of St. Augustine's (errant?) views on the passing of guilt in the human race.
It does entail, though, a level of participation in God's holiness (as He is the only source and possessor of holiness -- it isn't a created 'moral' quality) that seems somewhat unprecedented Biblically (on the human level: the buildings of Temple and Tabernacle had already partook of the incarnational grace): this really isn't bothersome, though, as Mary is no ordinary human, even though she is just like us ontologically. She is fully human, not a demi-god(dess). But, she is the "birth-giver to God" (a more precise translation of Theotokos than "mother of God"), a status, role, and honor that no other human being will ever have.
Two possibilities arise from this: either Mary never sinned and was cleansed from ritual impurity by the direct action of God or she sinned but was forgiven. I'm not sure it really matters: however, we often like to point out the moral failings of Biblical characters, so that we can relate. Why, though, should that be our default position? Holiness, righteousness, etc. are about participation with God through His grace, not about moral strength/willpower. There certainly is an element of struggle (ascesis), but that should spur us on to imitation, not depress us: Mary is human like us, her faithfulness to God is what we should emulate; not whether or not she sinned.
At any rate, there does seem to be an underlying theological principle that the Mother is the symbol of the Bride: in other words, if we want to know what the Church is to be, we need to look to Mary. The Church is to be a spotless virgin; Mary was a spotless virgin. The Church is to obey her Lord, for she is His agent of Life in the world; Mary, in her act of obedience ("let it be according to your word"), brings the Life into the world. The list could go on. The most telling moment, though, is what Mary says after asking her Son to help at Cana: she turns to those in charge of the festivities and says, "Do whatever He tells you." Whatever role Mary has (and, for those who follow the Regulative Principle of Worship, "all generations shall henceforth call me blessed"), she should be listened to here: she always will point us towards obedience, for the Life of the world.