First, the papal bull dogmatizing the assumption explicitly states that her body was assumed into heaven after her death. Second, Enoch and Elijah were taken up into heaven without dieing. http://forums.catholic.com/showthread.php?t=232146
Pope Pius XII, in Munificentissimus Deus, does not define that the Blessed Virgin died, only that she "completed the course of her earthly life" (MD 44). Only the specific definition of the Assumption in paragraph 44 of Munificentissimus Deus is the infallible proclamation; the rest of the document is commentary. Although many theologians believe that the Virgin Mary did die, the Church is officially silent on the matter.
So only certain parts of an infallible statement are infallible? I apologize for the long post.
5. Now God has willed that the Blessed Virgin Mary should be exempted from this general rule. She, by an entirely unique privilege, completely overcame sin by her Immaculate Conception, and as a result she was not subject to the law of remaining in the corruption of the grave, and she did not have to wait until the end of time for the redemption of her body.
17. In the liturgical books which deal with the feast either of the dormition or of the Assumption of the Blessed Virgin there are expressions that agree in testifying that, when the Virgin Mother of God passed from this earthly exile to heaven, what happened to her sacred body was, by the decree of divine Providence, in keeping with the dignity of the Mother of the Word Incarnate, and with the other privileges she had been accorded. Thus, to cite an illustrious example, this is set forth in that sacramentary which Adrian I, our predecessor of immortal memory, sent to the Emperor Charlemagne. These words are found in this volume: "Venerable to us, O Lord, is the festivity of this day on which the holy Mother of God suffered temporal death, but still could not be kept down by the bonds of death, who has begotten your Son our Lord incarnate from herself."
18. What is here indicated in that sobriety characteristic of the Roman liturgy is presented more clearly and completely in other ancient liturgical books. To take one as an example, the Gallican sacramentary designates this privilege of Mary's as "an ineffable mystery all the more worthy of praise as the Virgin's Assumption is something unique among men." And, in the Byzantine liturgy, not only is the Virgin Mary's bodily Assumption connected time and time again with the dignity of the Mother of God, but also with the other privileges, and in particular with the virginal motherhood granted her by a singular decree of God's Providence. "God, the King of the universe, has granted you favors that surpass nature. As he kept you a virgin in childbirth, thus he has kept your body incorrupt in the tomb and has glorified it by his divine act of transferring it from the tomb."
20. However, since the liturgy of the Church does not engender the Catholic faith, but rather springs from it, in such a way that the practices of the sacred worship proceed from the faith as the fruit comes from the tree, it follows that the holy Fathers and the great Doctors, in the homilies and sermons they gave the people on this feast day, did not draw their teaching from the feast itself as from a primary source, but rather they spoke of this doctrine as something already known and accepted by Christ's faithful. They presented it more clearly. They offered more profound explanations of its meaning and nature, bringing out into sharper light the fact that this feast shows, not only that the dead body of the Blessed Virgin Mary remained incorrupt, but that she gained a triumph out of death, her heavenly glorification after the example of her only begotten Son, Jesus Christ-truths that the liturgical books had frequently touched upon concisely and briefly.
22. These words of St. John Damascene agree perfectly with what others have taught on this same subject. Statements no less clear and accurate are to be found in sermons delivered by Fathers of an earlier time or of the same period, particularly on the occasion of this feast. And so, to cite some other examples, St. Germanus of Constantinople considered the fact that the body of Mary, the virgin Mother of God, was incorrupt and had been taken up into heaven to be in keeping, not only with her divine motherhood, but also with the special holiness of her virginal body. "You are she who, as it is written, appears in beauty, and your virginal body is all holy, all chaste, entirely the dwelling place of God, so that it is henceforth completely exempt from dissolution into dust. Though still human, it is changed into the heavenly life of incorruptibility, truly living and glorious, undamaged and sharing in perfect life." And another very ancient writer asserts: "As the most glorious Mother of Christ, our Savior and God and the giver of life and immortality, has been endowed with life by him, she has received an eternal incorruptibility of the body together with him who has raised her up from the tomb and has taken her up to himself in a way known only to him."
28. Thus, during the earliest period of scholastic theology, that most pious man, Amadeus, Bishop of Lausarme, held that the Virgin Mary's flesh had remained incorrupt-for it is wrong to believe that her body has seen corruption-because it was really united again to her soul and, together with it, crowned with great glory in the heavenly courts. "For she was full of grace and blessed among women. She alone merited to conceive the true God of true God, whom as a virgin, she brought forth, to whom as a virgin she gave milk, fondling him in her lap, and in all things she waited upon him with loving care."
35. In like manner St. Francis of Sales, after asserting that it is wrong to doubt that Jesus Christ has himself observed, in the most perfect way, the divine commandment by which children are ordered to honor their parents, asks this question: "What son would not bring his mother back to life and would not bring her into paradise after her death if he could?" And St. Alphonsus writes that "Jesus did not wish to have the body of Mary corrupted after death, since it would have redounded to his own dishonor to have her virginal flesh, from which he himself had assumed flesh, reduced to dust."
39. We must remember especially that, since the second century, the Virgin Mary has been designated by the holy Fathers as the new Eve, who, although subject to the new Adam, is most intimately associated with him in that struggle against the infernal foe which, as foretold in the protoevangelium, would finally result in that most complete victory over the sin and death which are always mentioned together in the writings of the Apostle of the Gentiles. Consequently, just as the glorious resurrection of Christ was an essential part and the final sign of this victory, so that struggle which was common to the Blessed Virgin and her divine Son should be brought to a close by the glorification of her virginal body, for the same Apostle says: "When this mortal thing hath put on immortality, then shall come to pass the saying that is written: Death is swallowed up in victory."
42. We, who have placed our pontificate under the special patronage of the most holy Virgin, to whom we have had recourse so often in times of grave trouble, we who have consecrated the entire human race to her Immaculate Heart in public ceremonies, and who have time and time again experienced her powerful protection, are confident that this solemn proclamation and definition of the Assumption will contribute in no small way to the advantage of human society, since it redounds to the glory of the Most Blessed Trinity, to which the Blessed Mother of God is bound by such singular bonds. It is to be hoped that all the faithful will be stirred up to a stronger piety toward their heavenly Mother, and that the souls of all those who glory in the Christian name may be moved by the desire of sharing in the unity of Jesus Christ's Mystical Body and of increasing their love for her who shows her motherly heart to all the members of this august body. And so we may hope that those who meditate upon the glorious example Mary offers us may be more and more convinced of the value of a human life entirely devoted to carrying out the heavenly Father's will and to bringing good to others. Thus, while the illusory teachings of materialism and the corruption of morals that follows from these teachings threaten to extinguish the light of virtue and to ruin the lives of men by exciting discord among them, in this magnificent way all may see clearly to what a lofty goal our bodies and souls are destined. Finally it is our hope that belief in Mary's bodily Assumption into heaven will make our belief in our own resurrection stronger and render it more effective.