The instruction of the CDF, Dignitas Personae, takes up the question of what could be done with the frozen embryos that are already in existence. It rejects the use of these embryos for research or for the treatment of disease because this would be contrary to their dignity as persons. It further takes up the question of their being given to infertile couples as a treatment for infertility, and rejects this as ethically wrong:
The proposal that these embryos could be put at the disposal of infertile couples as a treatment for infertility is not ethically acceptable for the same reasons which make artificial heterologous procreation illicit as well as any form of surrogate motherhood;[note: Cf. Congregation for the Doctrine of the Faith, Instruction Donum vitae, II, A, 1-3: AAS 80 (1988), 87-89.] this practice would also lead to other problems of a medical, psychological and legal nature.
Finally, it considers the proposal of adopting these embryos precisely to give them a chance to live:
It has also been proposed, solely in order to allow human beings to be born who are otherwise condemned to destruction, that there could be a form of “prenatal adoption”. This proposal, praiseworthy with regard to the intention of respecting and defending human life, presents however various problems not dissimilar to those mentioned above.
Here the document waffles, speaking vaguely about "various problems not dissimilar to those mentioned above." Since the moral objections reference in Donum Vitae do not apply to this situation, the most reasonable way to understand the "various problems not dissimilar to those mentioned above" is in reference to the "problems of a medical, psychological and legal nature" rather than to a moral evil. Thus we should read this paragraph along the following lines: the document neither intends, nor in fact does condemn embryo adoption as wrong in itself, yet expresses practical concerns of prudence regarding the issue. This interpretation is supported also by the statement of the president of the Pontifical Academy for Life, Archbishop Rino Fisichella, that the issue of embryo adoption is "still an open issue," and that of the U.S. Bishops Conference, saying that "The document raises cautions or problems about these new issues but does not formally make a definitive judgment against them" (Questions and Answers on 'Dignitas Personae'). The principal of these concerns is probably that of material cooperation with those involved in illicit use of embryos, and the potential scandal linked with it, as is argued by John Grabowski and Christopher Gross in an article in the National Catholic Bioethics Quarterly, Dignitas personae and the Adoption of Frozen Embryos – A New Chill Factor?
In the end, I am inclined to think that if in fact the motive is saving the lives of persons, and known to be such, the witness provided by this more than outweighs the possible evil of material cooperation. Moreover, the resistance to prenatal adoption of frozen embryos is seen by not a few to belie the Church's own position on the human dignity of such embryos, and so is a scandal in its own right. I think, therefore, that while not ignoring the concerns alluded to by the CDF, one should see the choice of prenatal adoption of embryos that would otherwise perish as a good and positive choice.
Janet Smith wrote an article last year in favor of embryo adoption–Adopting Embryos: Why Not?–and while not rigorously argued, and lacking in precision, I think most of her instincts are correct. I also recommend a longer, more academic article by Stephen Napier, Moral Justification and Human Acts: A Reply To Christopher Oleson, which examines closely the text of Dignitas personae and Donum vitae, and argues in favor of the legitimacy of embryo adoption.