The concept of limbo has never been a de fide element of the Catholic faith, though there have been times and places where it was taught more forcefully than others. Aquinas was quite sure of its existence, but his version of limbo is basically heaven without conscious awareness of being in heaven--something that seems somewhat apt given the souls that are supposed to populate the place.
But now the Vatican is trying to put limbo deeper into limbo, and this will seem a golden opportunity to those who claim that the Magisterial teachings of the Church are not so magisterial as some of us like to believe. There are those who point to practical teachings on things such as usury, slavery, and the like, note how these practical teachings have developed through time, and used this information to draw the erroneous inference that the Magisterial teachings themselves are open to rather drastic change.
Although this inference is false, it is dangerous to the Church because not only do many folks outside the Church believe it to be true, but the belief outside the Church is so strong that it has begun to affect the beliefs of folks within the Church as well. We already live in times when folks who call themselves Catholic feel that there is no big deal to rejecting Magisterial teachings on matters like birth control and abortion, so it's not much of a stretch for minds like that to read the development of doctrine as changes in teachings.
The concept of limbo was never more than a theological tool for exploring certain sorts of important themes, such as the necessity of baptism for salvation, the nature of God's love, and the extent of God's mercy. Similar sorts of theological themes have taken its place, and will probably also wax and wane. In Aquinas' day infant mortality was much higher than it is today, at least in those areas where this kind of theological speculation takes place, and some sort of examination of what happens to the children who die before baptism was probably motivated by much more than a worry about the necessity of the sacrament for salvation. If nothing else it reassured a population that lived in constant fear of death that those who die in infancy are not necessarily consigned to the flames of hell--they are in fact, Aquinas says, among the elect, but they are not capable of achieving the Beatific Vision because, in his view, their intellects are not developed enough for that. This is not much of a starter these days, but in the 13th century a comfort such as that was comfort indeed.
It is tempting to suggest that, once out of their bodies, the souls of infants who die before baptism will continue to develop and will eventually see God along with the rest of us. But to think this way is to forget about the doctrine of the resurrection, according to which we will be raised bodily, not just spiritually. That teaching is a de fide element of the Magisterium, and it is not going to go away, however much heretical folks like John Spong think that it must. Of course it is quite impossible to know what sorts of bodies we will have at the Resurrection, but we cannot assume that they will all be endowed with the same capacities, or even capacities of the sort that we have now.
I'm not sorry to see the doctrine of limbo put onto a shelf, though I'm not convinced that it is really all that bad of an idea. What worries me more is what our debauched culture and under-catechized Church will make of it all. Perhaps not much will be made of it--perhaps few are even paying attention. That would probably be a good thing.