Given the serious commandment of kibud av' v'em (honoring father and mother), ought a person attend the interfaith wedding of his or her parent, even if they don't "agree" with intermarriage per se?

As you haven't provided any details regarding this situation - i.e. the level of people's observance and their ages, the nature of the relationship between parent and child, the possibility of future conversion, whether the wedding ceremony will include other religious rites, the intent of the couple to have more children etc. - I will first answer generally. When there is a possible conflict between one's Jewish observance and the mitzvah of honouring one's parents, Jewish law is clear - since both children and parents are commanded to follow God's Torah, we must follow the Torah (Bava Metzia 32a). In the ideal situation, of course, there should be no conflict, as parents and children should both reflect and epitomize Jewish observance, concomitantly bringing the family together in loving harmony. Unfortunately, however, we often have cases like the one above, though the shoes are usually on the opposite feet.

While the parent here is not telling his child to transgress a specific law, the celebration of a marriage is obviously a very significant event, with the ceremony reflecting the bride and groom's values. Due to the significant danger that intermarriage poses for the future of the Jewish people, it would seem highly inappropriate to attend such a wedding, even if they won't be having children. The child should sit down with his parent and other family members and explain that while he deeply loves and honours such parent, going to such a wedding would be too painful for him. It is important to stress that an adult child may not condone everything a parent does, but can still love and respect such parent.

Nevertheless, there may be some grey in what might reflexively seem like a black and white issue. For example, depending on the family dynamics and the risk of deeply alienating the parent or other family members, I could envision a situation whereby the child would attend the reception, but skip the ceremony under all circumstances, especially if it involved other religious rites. This type of compromise would send the proper message about the importance of the Jewish tradition, while clearly showing that the child still greatly values his relationship to his parent, even in trying circumstances. There is no easy answer to this very difficult question, but by respectfully observing both the letter and spirit of Jewish law, while continuing to show love to one’s parent, it may be possible to both maintain one’s family’s love and respect, while remaining true to one’s deepest values.

One last point - while the details here may lead to somewhat different conclusions, the overall thrust of my response is an example of the difference between Jewish and modern liberal values, in which acceptance of almost any viewpoint is valid and defining right and wrong is generally frowned upon. There are objective criteria in defining such Jewish values and when these values conflict with contemporary values, which can change over a short period, Jewish tradition and history teaches us to follow eternal Jewish values. How to apply them, of course, is a bit more difficult, but we pray that God guides us in this pursuit.