Over at Fr. Zuhldorf's blog, he provides a link to a translated piece (coming originally via the Cafeteria is Closed) wherein Cardinal Christoph Schoenborn discusses ad orientem and versus populum: Ad orientem versus at the Viennese Cafeteria
Fr. Zuhlsdorf gives some interesting commentary through the piece.
I believe that one question which fails to be given enough emphasis when this issue arises in this kind of context (I speak of the Cardinal, not Fr. Zuhlsdorf) is that practical consideration, apart from the question of theology (which indeed would find neither orientation inadmissable inherently), of how this shift to versus populum liturgies has been understood and implemented by faithful and priests alike and the fruits that have come from that in our current secular and ecclesial culture.
A great problem in the liturgy today is the over-emphasis upon the communal, as well as a fundamental misperception of the nature of the Christian community in the liturgy and the ultimate horizontalization of the liturgy. That being the case, a pragmatic question enters in addition to the questions of our longstanding liturgical tradition of ad orientem and that is this: which practice can best serve as a corrective to this problem, or help prevent the problem in the first place, and also best stand in continuity with our tradition and acting in unison with the various ritual traditions of the Church? The answer is clearly ad orientem.
While versus populum is not inherently wrong, and could be received properly under the proper conditions and disposition of both the priest and the laity, we can ask, what have been the effects as we've found them in our climate and conditions?
We can argue for intellectual catechesis as the answer, but as we all know, in liturgical matters, while catechesis is great, liturgical praxis itself is often the best form of catechesis. This is particularly the case when you are dealing with two warring liturgical theologies. In such an instance, what is needed is enforcement and actions which speak of the message the Church wishes to get across.