Many people have begun to ask about the document expected at some point from Ecclesia Dei which will give guidelines for understanding the proper intent and interpretation of Summorum Pontificum.
While we yet await the release of the document, it strikes me as important to note that one cannot turn to anyone better to understand what that document will likely contain than Cardinal Castrillon Hoyos, the President of the Commission given the competency over this.
In that vein, those interested in gaining an insight into at least some of the content of these clarifications should pay particular attention to statements give by Cardinal Castrillon Hoyos in this interview and in the introduction to the FSSP training DVD which touches upon such matters as the intended scope of the Motu Proprio, the matter of what constitutes a "stable group" and so on.
Of course, interviews do not compare to an official document, but still, it is worth taking note of his commentary on the motu proprio as found there.