In Caritas in veritate there is a reference to Humanae vitae.
Remember that the Pope is addressing himself ad intra and ad extra, to the Church herself and to the world at large.
15. Two further documents by Paul VI without any direct link to social doctrine — the Encyclical Humanae Vitae (25 July 1968) and the Apostolic Exhortation Evangelii Nuntiandi (8 December 1975) — are highly important for delineating the fully human meaning of the development that the Church proposes. It is therefore helpful to consider these texts too in relation to Populorum Progressio.
The Encyclical Humanae Vitae emphasizes both the unitive and the procreative meaning of sexuality, thereby locating at the foundation of society the married couple, man and woman, who accept one another mutually, in distinction and in complementarity: a couple, therefore, that is open to life. This is not a question of purely individual morality: [The Holy Father is waging a battle against relativism.] Humanae Vitae indicates the strong links between life ethics and social ethics, ushering in a new area of magisterial teaching that has gradually been articulated in a series of documents, most recently John Paul II’s Encyclical Evangelium Vitae. The Church forcefully maintains this link between life ethics and social ethics, fully aware that “a society lacks solid foundations when, on the one hand, it asserts values such as the dignity of the person, justice and peace, but then, on the other hand, radically acts to the contrary by allowing or tolerating a variety of ways in which human life is devalued and violated, especially where it is weak or marginalized.” …
 Cf. nos. 8-9: AAS 60 (1968), 485-487; Benedict XVI, Address to the participants at the International Congress promoted by the Pontifical Lateran University on the fortieth anniversary of Paul VI’s Encyclical “Humanae Vitae”, 10 May 2008.
 Cf. Encyclical Letter Evangelium Vitae (25 March 1995), 93: AAS 87 (1995), 507-508.
 Ibid., 101: loc. cit., 516-518.
What to take away from this?
This is what I think is going on.
Benedict XVI is obviously going to make a reference to this vastly important part of the Paul VI’s magisterium, since the whole encyclical looks back to him.
Benedict is also offering a challenge to those who think that they can diminish the important of human life and its dignity and the natural act by which human life is conceived when considering how we approach social ills.
He is reminding the world … and Catholics within the Church! … that you cannot diminish these elements, explained in Humanae vitae, and honestly think that you are for true justice.