[More home team rooting, here -- our paths crossed at Notre Dame, where we both did our doctoral work in trinitarian theology, and he is now at the School of Theology at St. John's University in Collegeville, where I did my master's work, long ago (he also teaches in the joint College of St. Benedict/St. John's University undergraduate theology department).]
My take: yes, it's an unusual choice. No retired Irish anything. No Boston.
First off, he's 45, young for a theologian and young for a post like the Vatican. On the other hand, there's a confirmation process involved, and "young" leaves less of a paper trail for folks to pounce on, as Obama knows well. (There's a reason that most elementary school productions of the Nativity story ignore the fifth-grade spelling stars and reach into the second grade to cast the Blessed Virgin...)
He's comfortable in several languages, a party-trick in the U.S., essential for the Holy See.
Díaz is clearly in the Obama camp, on record as donating to the Obama campaign and vocal as a speaker for the progressive group Catholics in Alliance for the Common Good. But make no mistake, he's clearly pro-life, part of the newly-energized movement of pro-life Catholics, who find Obama's policies to reflect the kind of social order this is committed to community and justice in a way that moves beyond the zero-sum calculus of the old left and the neo-con right.
Díaz' theological work is in the early stages but it would be good to remember that detailed theological discussion is not the "work" of a U.S. ambassador, even one posted to the Vatican -- there's just too much crying need out there. (Daydreaming about Díaz and Ratzinger going mano-a-mano over the doctrine of grace? It ain't gonna happen.)
What's important, for this appointment, are the broad themes to which he returns, over and over, in his work. His book, "On Being Human," brings together the particular story of experientially-focused Hispanic theological insights with Karl Rahner's broad and generous story of humanity in search of God. It's THIS broad framework, this pattern of movement 'from disparate roots to common purpose,' that is well-suited to the needs of the Obama administration as it works in concert with the global concerns that face Benedict XVI everyday.
If Miguel Díaz can effectively serve as the face of the serious Catholic committment to a more just and inclusive world, one that is particularly attuned to the needs of the most vulnerable among us, he will do well. Can he work "in concert" with the Vatican? And, more pointedly, will the competing forces in U.S. Catholicism allow him to do the good work to which the Obama administration has called him?
I hope so. I pray so.