My Motu Proprio theory
Well, as lots of people have a fuss over the Motu Proprio that's supposed to come out soon, allowing the "Tridentine"* rite to be celebrated by any obedient priest who so desires...I'll just drop my two cents in.
My theory is that the Motu Proprio is to be released once either: a) the SSPX caves in on something important, like the validity of several Vatican II documents, or b) a significant liberal group or clique starts obeying the Pope. However, the former is more likely.
Let's view it in a political sense. The MP is something both liberal and traditionalist are interested in - the latter is excited, the former worried. Merely saying there is a MP in the works can be used as a goad. The traditionalists can be told: "Obey us or this Motu Proprio stays in a desk at the Vatican." The liberals can be told: "So, you don't want the Motu Proprio coming out, huh? Well, then, do what we say!"
Of course, traditionalists are more likely to be affected by this, as liberals will usually just abandon ship for the Nutty Church of Sumpin'. So I expect some valuable concession by the SSPX will precede the Motu Proprio's release.
------------------------------------------------
Of course, this does not deal with the religious or cultural effects the MP could have, nor does it mention that the Holy Spirit could throw a curveball (Cf. Pope Vigilius) that defies political common sense. It's just a theory on the subject. After all, part of the Pope's duty is politics, and the MP is political as well as religious in significance.
*I put quotation marks around "Tridentine" because many people do not like the name, and, although it is the most common word used for the pre-Vatican II mass, it isn't quite accurate. The Tridentine rite did not originate in the Council of Trent. It was an Italian rite that the Council chose as a worldwide standard.
