Here's a great article from Homiletics and Pastoral Review that explains this concept of "tenenda" in terms of Ordinatio Sacerdotalis and it's "ex cathedra" status:
http://www.catholicculture.org/library/view.cfm?recnum=835
Sorry--it's kind of long--but I think it's helpful here.
There doesn't seem to be any peer review with Homiletic and Pastoral Review, so the article quality varies quite a bit. In this article, the giveaway is in the first sentence:
In the theological discussion of Ordinatio Sacerdotalis there has been a dearth of argumentation for the position that the Apostolic Letter contains an ex cathedra definition.
However, nowhere does the article state that the reason for this dearth (lack) is that the Church's
magisterium has repeatedly clarified that
Ordinatio Sacerdotalis is
not ex cathedra.
Here is what Cardinal Ratzinger wrote
in 1995:
In this case, an act of the ordinary papal Magisterium, in itself not infallible, witnesses to the infallibility of the teaching of a doctrine already possessed by the Church.
Here is what Cardinal Bertone wrote
in 1997:
It must be stressed then that in the Encyclicals Veritatis splendor and Evangelium vitae and in the Apostolic Letter Ordinatio sacerdotalis, the Roman Pontiff intended, though not in a solemn way, to confirm and reaffirm doctrines which belong to the ordinary, universal teaching of the Magisterium, and which therefore are to be held in a definitive and irrevocable way.
and:
If we were to hold that the Pope must necessarily make an ex cathedra definition whenever he intends to declare a doctrine as definitive because it belongs to the deposit of faith, it would imply an underestimation of the ordinary, universal Magisterium, and infallibility would be limited to the solemn definitions of the Pope or a Council, in a way that differs from the teaching of Vatican I and Vatican II, which attribute an infallible character to the teachings of the ordinary, universal Magisterium.
Here is another article that claims that
Ordinatio Sacerdotalis is
ex cathedra. At least this person has the guts to acknowledge directly that he is dissenting from Cardinal Ratzinger:
With all due respect to Cardinal Ratzinger, if it walks like a duck, and quacks like a duck...guess what?
However, my money is on Cardinal Ratzinger, now Pope Benedict XVI, and the Church's magisterium. Dissent from them at your own risk.