Another thought which struck me was the fine example of tolerance and co-operation shown by the Chinese Catholics in such difficult and highly-charged circumstances. With our relatively easy civic freedom, perhaps we could see this as a model for co-operation with the Society of St Pius X? Please don't misunderstand me: I know that the comparison is not exact and I am certainly not suggesting that either side is the equivalent of a "patriotic Church" but if the Catholics in China can work together despite such important and emotive differences, should it be so hard for us to do so?
I originally wrote the following as a comment to Fr Tim's post, and then realised it was a bit long for a comment, so am posting it as a new post and expanding it. Read Fr Tim's full post first
The suggestion of a certain parallel between the conversation with the Society of St Pius X and the situation of the Catholic Church in China ("underground" and "patriotic") is, I think, closer than you suggest if we look at it as an exercise of the "ministry of unity" of the Successor Peter.
I came rather late to reading Pope Benedict's Letter to Chinese Catholics, but there seemed to me to be a striking similarity between the principle underlying that initiative and the principle underlying the initiative towards the Society of St Pius X (and, therefore indirectly, the initiative of Summorum Pontificum - and, in my view, this consideration of unity is at least as important as Liturgical ones, if not more so, to understanding Summorum Pontificum).
As I read the Letter to Chinese Catholics (and an associated Q+A released, I think, to counter distortion of the original Letter by the Chinese authorities), I was struck by its concern with regard to the unity of the Church and the vital necessity of the communion of the Bishop with the Successor of Peter to assure that unity, so much so that I read that concern as being the most important theological principle underlying the Letter.
In both the Chinese and St Pius X contexts, I think we can see Pope Benedict doing everything that he can to try and restore that unity based on the communion of the Bishop with the Holy See where it has suffered injury - fundamentally the same enterprise undertaken in two very different historical/political contexts. In the one context, the Letter to Chinese Catholics encourages communion between the "underground" and "patriotic" churches, in exactly the way described in Fr Tim's post and achieved in such a profound way at the funeral liturgy of a Bishop, so that communion between their Bishops can grow; and in the other the Liturgical intitative of Summorum Pontificum is about encouraging increasing communion between those attached to "ordinary" and "extraordinary" forms to help bring about communion of the Bishops (and religious superiors/communities) concerned. This is why I consider the agenda of "mutual enrichment" of Pope Benedict's letter accompanying Summorum Pontificum to be so important, as it represents the aspect of building of ecclesial communion. Sadly, it is the aspect of the initiative of Summorum Pontificum that has been almost totally neglected.
I think the example of communion between underground and patriotic Catholics at the funeral of Bishop James Lin Xili is a very striking and profound witness to the unity of the Church that Pope Benedict seeks to encourage, and Fr Tim is quite right to point it out as a model for the dialogue with the Society of St Pius X.
PS: The tangible concern for the unity of the Church that I am suggesting here has, I think, a distinctly Patristic feel to it; we in the West have perhaps lost sight of it since the Reformation in a concern for the truth of the content of the faith (not that the two are contradictory, but rather go together). It also prompts a rather interesting reflection on the nature and definition of ecumenical activity - I recall there being comment at the time of the lifting of the excommunications of the Bishops of the Society of St Pius X that it was fortuitious that that action coincided with the octave of prayer for Christian unity.