Perhaps the official dogmatic proclamation, Ineffibilis Deus, will shed some light on your question(http://www.newadvent.org/library/docs_pi09id.htm):
“For it was certainly not fitting that this vessel of election should be wounded by the common injuries, since she, differing so much from the others, had only nature in common with them, not sin. In fact, it was quite fitting that, as the Only-Begotten has a Father in heaven, whom the Seraphim extol as thrice holy, so he should have a Mother on earth who would never be without the splendor of holiness.”

Given that I come at this as a former Roman Catholic and now differ quite substantially from RCs in my theological and historical perspectives on this issue, I’m not sure that I can give you a viewpoint that fully reflects RC sensibilities.

Those unfamiliar with liturgical calendars may find curious the fact that today is the Feast of the Immaculate Conception in the RC church and is a day of obligation in which RCs must attend Mass to avoid commission of a mortal sin. The Eastern Orthodox celebrate the Feast of St Anna’s Conception of the Theotokos tomorrow. The consider this a venerable feast of St Anna, permitting a relaxation of fasting rules (katalysis) for wine and oil, rather than as a feast of the Mother of God, which would permit katalysis for the consumption of fish. One may further note that the RC feast of the Immaculate Conception is held exactly 9 months prior to the Feast of the Nativity of the Theotokos whereas the EO’s celebration of St Anna’s Conception of Mary is “off” by one day. The feasts of Christ’s Conception (the Annunciation on March 25) and his Nativity are exactly 9 months apart in both RC and EO traditions. No other feasts of conception and nativity for any other saint are celebrated exactly 9 months off. I say this because it demonstrates a theological difference liturgically. If anyone knows when the dates for the celebration of Mary’s conception diverged, that would be very interesting.

One may also find interesting the fact that Thomas Aquinas, the great scholastic who forged many of the doctrines disputed by the Reformers and confirmed by the Council of Trent, believed quite strongly that Mary had personally committed venial sins.

Just some tidbits to ponder. I have this vague suspicion that, if Protestants were to take seriously their traditional scholastic view of original guilt, they might just speculate themselves into a belief in the necessity of the dogma of the Immaculate Conception.

Of course, that’s just my own speculation and, of course, Catholics would burn me like Hus and Protestants would burn me like Servetus.

Maybe I wasn’t clear; I don’t think I’m so awesome because I figured it out and all those nasty Catholics haven’t. I’m saved by grace through faith in Christ alone, and that’s not what the Catholic church teaches.

I was responding to Adam’s comment about how only people within the Roman or Orthodox church are saved. I’d say it’s the opposite. We get to disagree, and one of us has to be wrong. I simply want people to base their Christian faith in what the Bible has to say.

“It’s funny how from the beginning, the people who claim to love God most zealously are the ones who get most upset about His loving unsavory types.”

Adam, Chrish, please reread.