Mark Shea has a lively discussion going, speculating on whether or not Mary was sacramentally baptized. He is in favor of Mary not having been. The comments are up to 57 at the moment and I haven't read them all but rather briefly scanned through them.
One commenter has made these remarks in favor of Mary's baptism:
"Mary needed to baptised to receive the other sacraments (non baptized individuals can not validly receive any other sacrament). Mary could not have received the Eucharist with baptism......To hold the position that Mary never was baptized is to hold that she was never confirmed or received the Eucharist. If you do not know the theological principle stated above, look it up in any orthodox book on the Sacramental Theology.....The Immaculate Conception did not: Make Mary a Christian, because it did not give her the mark of baptism. It did not incorporate her into the Mystical Body of Christ."
As Mark noted, he seems very cocksure in speculation as if he were presenting dogma.
In all humility and as merely speculation, I would propose the following answer to the commenter (and maybe someone already has stated the same. As I said, I haven't read all the comments):
First of all, Mary wouldn’t require baptism because of her Immaculate Conception. Second she would be initiated into the life of Christ by the very fact that she is His Mother! One could say that Mary carrying Christ in the womb for 9 months (all the while receiving unlimited grace) would act as a sort of baptism. The problem with the commenter's logic is that he is comparing sinless and Immaculate Mary to the rest of sinful creation.
Now, Mark does bring up a good point that maybe Mary would have chosen sacramental baptism without needing it in order to be more like Christ who was baptized without the need for it. This would go hand in hand with the speculation of whether Mary died or not before her Assumption. The tradition in the East says that Mary chose to die in order to be more like Christ (yet her death did not corrupt her body) and the West traditionally says that she was assumed without death at the end of her life. The only thing about the baptism for Mary, is that it isn't anyway in Tradition (which is why I agree with Mark against it). Of course, arguments from silence aren't the best as well......isn't theology fun!