There's a lively discussion going on over at Fr Z over the extent to which the Dormition of Mary is part of the deposit of faith. It was stimulated, unfortunately by a rather arrogant and rude comment by an Eastern Rite Catholic, but has brought out some interesting points, not the least of which is the inability of many today to take proper note of the different between what we must believe, what we may believe and what we must not believe!
The commenters main message is that while Mary's death hasn't been formally defined, there is a lot of evidence from tradition for it - in the Eastern rite/Orthodox liturgy, in sacred art, and in the writings of the Church Fathers in particular.
The evidence for Our Lady's death
Some Eastern rite catholics in the debate have asserted that Westerners are ignoring their tradition. The problem is that there are a number of issues where Western and Eastern tradition diverge liturgically and in their traditions.
One example is St Mary Magdalene - the Eastern church view the various stories about women followers of Our Lord as being about different people. They reject for example the association of the sinful woman from Luke 7 as Mary Magdalene - whereas the traditional Latin Mass uses that text for St Mary Magdalene's feast day!
Another problem from the Western perspective is that the historical evidence one way or another is not strong - there are, as I've previously pointed out some traditions about the apostles gathering in Jerusalem for her death, but the written record of these is not early. There are private revelations going both ways (Elizabeth of Schonau saw a vision in which Our Lady did die; the Ven. Mary of Jesus of Agreda apparently saw the opposite).
Personally, I think the fact that the view that Our Lady didn't die arose very late, and runs counter to the monuments of tradition is fairly compelling. But the question is are we free to disagree on this?
A really excellent resource on questions like this is Dr Ott's Fundamentals of Catholic Dogma. First, whether or not Our Lady died is (potentially at least) a dogmatic fact:
"Dogmatic Facts (facta dogmatica). By these are understood historical facts, which are not revealed, but which are intrinsically connected with revealed truth, for example, the legality of a Pope or of a General Council, or the fact of the Roman episcopate of St. Peter. The fact that a defined text does or does not agree with the doctrine of the Catholic Faith is also, in a narrower sense, a "dogmatic fact." In deciding the meaning of a text the Church does not pronounce judgment on the subjective intention of the author, but on the objective sense of the text (D 1350 : sensum quem verba prae se ferunt). "
On such facts, people are free to debate until the Magisterium rules one way or another, which it clearly has not done in this case:
"Theological opinions are free views on aspects of doctrines concerning Faith and morals, which are neither clearly attested in Revelation nor decided by the Teaching Authority of the Church. Their value depends upon the reasons adduced in their favour (association with the doctrine of Revelation, the attitude of the Church, etc.).
A point of doctrine ceases to be an object of free judgment when the Teaching Authority of the Church takes an attitude which is clearly in favour of one opinion. Pope Pius XII explains in the Encyclical "Humani generis" (1950): "When the Popes in their Acts intentionally pronounce a judgment on a long disputed point then it is clear to all that this, according to the intention and will of these Popes, can no longer be open to the free discussion of theologians" (D 3013)."
There are however grades of theological certainty which go to how we should approach a particular assertion. This is how they are traditionally described:
"1. The highest degree of certainty appertains to the immediately revealed truths. The belief due to them is based on the authority of God Revealing (fides divina), and if the Church, through its teaching, vouches for the fact that a truth is contained in Revelation, one's certainty is then also based on the authority of the Infallible Teaching Authority of the Church (fides catholica). If Truths are defined by a solemn judgment of faith (definition) of the Pope or of a General Council, they are "de fide definita."
2. Catholic truths or Church doctrines, on which the infallible Teaching Authority of the Church has finally decided, are to be accepted with a faith which is based on the sole authority of the Church (fides ecclesiastica). These truths are as infallibly certain as dogmas proper.
3. A Teaching proximate to Faith (sententia fidei proxima) is a doctrine, which is regarded by theologians generally as a truth of Revelation, but which has not yet been finally promulgated as such by the Church.
4. A Teaching pertaining to the Faith, i.e., theologically certain (sententia ad fidem pertinens, i.e., theologice certa) is a doctrine, on which the Teaching Authority of the Church has not yet finally pronounced, but whose truth is guaranteed by its intrinsic connection with the doctrine of revelation (theological conclusions).
5. Common Teaching (sententia communis) is doctrine, which in itself belongs to the field of the free opinions, but which is accepted by theologians generally.
6. Theological opinions of lesser grades of certainty are called probable, more probable, well-founded (sententia probabilis, probabilior, bene fundata). [My view is that this is where Our Lady's death is located] Those which are regarded as being in agreement with the consciousness of Faith of the Church are called pious opinions (sententia pia). The least degree of certainty is possessed by the tolerated opinion (opimo tolerata), which is only weakly founded, but which is tolerated by the Church.
With regard to the doctrinal teaching of the Church it must be well noted that not all the assertions of the Teaching Authority of the Church on questions of Faith and morals are infallible and consequently irrevocable. Only those are infallible which emanate from General Councils representing the whole episcopate, and the Papal Decisions Ex Cathedra (cf. D 1839). The ordinary and usual form of the Papal teaching activity is not infallible. Further, the decisions of the Roman Congregations (Holy Office, Bible Commission) are not infallible.
Nevertheless normally they are to be accepted with an inner assent which is based on the high supernatural authority of the Holy See (assensus internus supernaturalis, assensus religiosus). The so-called "silentium obsequiosum." that is "reverent silence," does not generally suffice. By way of exception, the obligation of inner agreement may cease if a competent expert, after a renewed scientific investigation of all grounds, arrives at the positive conviction that the decision rests on an error."
Hope this is helpful....