Well, back in my (Catholic) seminary days in the 1960's, I was taught that Limbo was a theological invention that dates back to the time of St. Augustine (354-430 AD). Here's a quote from Fr. Richard McBrien's Harper Collins Encyclopedia of Catholicism:

In response to Pelagius, who taught that Baptism was not necessary for salvation, Augustine contended that unbaptized children who die are condemned to hell, though they do not suffer all its pains because they are not guilty of personal sin. Medieval theologians, wishing to mitigate the harshness of Augustine's position, postulated the existence of limbo. Limbo, they held, is a place or state where unbaptized persons enjoy a natural happiness, though they remain excluded from the Beatific Vision (seeing God). Some incorrectly identify this limbo with the hell of the Apostles' Creed where, according to tradition, Christ spent the interval between Good Friday and Easter Sunday.
...Neither officially defined nor abrogated by the Catholic Church, this theological postulate plays no role in contemporary Catholic theology.

In other words, it never was official doctrine of the Catholic Church, although that's what the nuns taught us in grade school. This does give me reason to have grave doubts about the infallibility of Sister Athanasia, O.P., who taught me in seventh grade at St. Rita School in Racine, Wisconsin. My seminary professors set me straight a couple years later.

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Here's a press release dated December, 2005, from the Catholic News Service:

VATICAN LETTER Dec-2-2005 (840 words) Backgrounder. xxxi

Closing the doors of limbo: Theologians say it was hypothesis
By Cindy Wooden
Catholic News Service

VATICAN CITY (CNS) -- An international group of Vatican-appointed theologians is about to recommend that the Catholic Church close the doors of limbo forever.

Many Catholics grew up thinking limbo -- the place where babies who have died without baptism spend eternity in a state of "natural happiness" but not in the presence of God -- was part of Catholic tradition. Instead, it was a hypothesis -- a theory held out as a possible way to balance the Christian belief in the necessity of baptism with belief in God's mercy. Like hypotheses in any branch of science, a theological hypothesis can be proven wrong or be set aside when it is clear it does not help explain Catholic faith.

Meeting Nov. 28-Dec. 2 (2005) at the Vatican, the International Theological Commission, a group of theologians led by Cardinal Joseph Ratzinger until his election as Pope Benedict XVI, completed its work on a statement regarding "the fate of babies who have died without baptism." A press release said the commission's statement would focus on the question "in the context of God's universal saving plan, the uniqueness of the mediation of Christ and the sacramentality of the church in the order of salvation."

U.S. Archbishop William J. Levada, president of the theological commission in his role as prefect of the Congregation for the Doctrine of the Faith, told Pope Benedict Dec. 1 that he hoped the statement would be published soon. Archbishop Levada said the question is important because "the number of babies not baptized has increased considerably" and the church knows that salvation "is only reachable in Christ through the Holy Spirit." But the church, "as mother and teacher," also must reflect on how God saves all those created in his image and likeness, particularly when the individual is especially weak "or not yet in possession of the use of reason and freedom," the archbishop said.

Redemptorist Father Tony Kelly, an Australian member of the commission, told Catholic News Service "the limbo hypothesis was the common teaching of the church until the 1950s. In the past 50 years, it was just quietly dropped. "We all smiled a bit when we were presented with this question, but then we saw how many important questions it opened," including questions about the power of God's love, the existence of original sin and the need for baptism, he said. "Pastorally and catechetically, the matter had been solved" with an affirmation that somehow God in his great love and mercy would ensure unbaptized babies enjoyed eternal life with him in heaven, "but we had to backtrack and do the theology," Father Kelly said.

A conviction that babies who died without baptism go to heaven was not something promoted only by people who want to believe that God saves everyone no matter what they do. Pope John Paul II believed it. And so does Pope Benedict. In the 1985 book-length interview, "The Ratzinger Report," the future Pope Benedict said, "Limbo was never a defined truth of faith. Personally -- and here I am speaking more as a theologian and not as prefect of the congregation -- I would abandon it, since it was only a theological hypothesis. "It formed part of a secondary thesis in support of a truth which is absolutely of first significance for faith, namely, the importance of baptism," he said.

In "God and the World," published in 2000, he said limbo had been used "to justify the necessity of baptizing infants as early as possible" to ensure that they had the "sanctifying grace" needed to wash away the effects of original sin. While limbo was allowed to disappear from the scene, the future pope said, Pope John Paul's teaching in the "Catechism of the Catholic Church" and the encyclical "The Gospel of Life" took "a decisive turn." Without theological fanfare, Pope John Paul "expressed the simple hope that God is powerful enough to draw to himself all those who were unable to receive the sacrament," the then-cardinal said.

Father Kelly said turning away from the idea of limbo was part of "the development of the theological virtue of hope" and reflected "a different sense of God, focusing on his infinite love." The Redemptorist said people should not think the changed focus is a lightweight embrace of warm, fuzzy feelings. "The suffering, death and resurrection of Christ must call the shots," he said. "If Christ had not risen from the dead, we never would have thought of original sin," because no one would have needed to explain why absolutely every human needed Christ's salvation. The fact that God loves his creatures so much that he sent his Son to die in order to save them means that there exists an "original grace" just as there exists "original sin," Father Kelly said. The existence of original grace "does not justify resignation," or thinking that everyone will be saved automatically, he said, "but it does justify hope beyond hope" that those who die without having had the opportunity to be baptized will be saved.

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In response to the question, "What else did they just 'make up' and claim it was an infallible doctrine?" MMario sez:

marriage as a religious rather then civil action; celibacy for priests; confession; and virtually every facet of organized Christian religion other then the eucharist and baptism.

Yeah, MMario, but most of those are practices or teachings, not infallible dogma. The infallible stuff is mostly what's in the Creed, which is shared by most Christian religions and dates back to 325 AD.
-Joe Offer-