An article here on Catholic Exchange today explains succinctly yet eloquently the history of the Church's teaching on the role of Mary in the salvation of the human race.
For many people -- including, as I understand it, Joseph Cardinal Ratzinger, before he became Pope Benedict XVI -- this title was considered problematic for the simple reason that it is too easily subject to misunderstanding and misinterpretation. All Marian dogmas developed in relationship to our deepening understanding of the person, nature, and work of Christ. As the mother of the divine person, Christ, she herself was not (and has never been considered) divine by nature. And yet she is rightly called the Theotokos, God-bearer, because in giving birth to Christ, she in fact gave birth to God Incarnate.
It is very, very important to keep in mind that, even if the Church were to pronounce Mary "Co-Redemptrix" by virtue of her role in God's plan of salvation, the "Co-" would not signify -- could never signify -- that Mary shared an equal role with Christ, the Redeemer of the World. (As one person in the CE article commented, a "co-star" or "co-pilot" does not have the same level of importance as the star and pilot, respectively. It would be sacraligious to suggest that the "Trinity" could ever become a "Quadrinity."
Rather (as I understand it), those who are asking the pope to define this particular dogma seek to acknowledge that, from the beginning, Mary was to be the channel by which God's plan of salvation was realized. From all eternity, God determined that Mary would be the woman uniquely called, and distinctively prepared, to be the Ark of the New Covenant, the New Eve, the Mother of the Church.
None of this by her own merit (as she herself would no doubt tell you), but all of it sheerly by God's grace -- the same grace that infuses us with divine life in the Eucharist, and will utterly restore us to perfection in heaven. Mary was the first to experience this perfection ... and as Mother of the Redeemer, she is able to intercede for us no other mother can.