I haven't wanted to prejudge Mel Gibson's film on Jesus, nor have I wanted to judge him based on his affiliation with a sect that rejects Vatican II, nor based on the anti-Semitism of his father. I've gotten many concerned emails from Jewish friends on all of the latter topics, and have basically ignored them, especially those that would judge him based on the nutty views of his dad.
The following, however, is at least troubling and deserves further explanation:

'YOU'RE GOING to have to go on record. The Holocaust happened, right?" Peggy Noonan asks of Mel Gibson in the Reader's Digest for March.
Gibson: "I have friends and parents of friends who have numbers on their arms. The guy who taught me Spanish was a Holocaust survivor. He worked in a concentration camp in France. Yes, of course. Atrocities happened. War is horrible. The Second World War killed tens of millions of people. Some of them were Jews in concentration camps. Many people lost their lives. In the Ukraine, several million starved to death between 1932 and 1933. During the last century, 20 million people died in the Soviet Union."

There are some serious problems with this statement, include the gratuitous lumping of the Holocaust with other tragedies that were not relevant in context, that suggest an aggressive hostility to Noonan's question (the question itself would seem a bit strange, but for the fact that Gibson's father is a Holocaust denier), and at best a desire to put the Holocaust into "context". But here's the really troubling part: "The Second World War killed tens of millions of people. Some of them were Jews in concentration camps." I'm no expert on Holocaust "revisionism" (denial), but I've read enough about it to know that this part of Gibson's statement sounds a heck of a lot like what the deniers say: instead of stating the historically obvious, that there was a systematic campaign to exterminate Jews in death camps, they say that Jews were merely among the many victims who suffered and died in concentration camps; maybe they suffered slightly more than others, but that's about it.
Indeed, Gibson is skirting pretty close to "Holocaust denial." Here is what the Nizkor.org website says about the topic:

The question [of whether the Institute for Historical Review, the leading Holocaust denial group, denies the Holocaust] appears to turn on IHR's Humpty-Dumpty word game with the word Holocaust. According to Mark Weber, associate editor of the IHR's Journal of Historical Review, "If by the `Holocaust' you mean the political persecution of Jews, some scattered killings, if you mean a cruel thing that happened, no one denies that. But if one says that the 'Holocaust' means the systematic extermination of six to eight millions Jews in concentration camps, that's what we think there's not evidence for." That is, IHR doesn't deny that the Holocaust happened; they just deny that the word "Holocaust" means what people customarily use it for.

Now compare this to what Gibson said. The Holocaust happened. He has friends and parents of friends with numbers on their arms. A guy who taught him Spanish had a tatoo, was a survivor of the camps (not clear if he was Jewish). Atrocities happened. Millions were killed. Some of them were Jews in concentration camps. Note what this leaves out: that Jews were murdered in death camps, and were murdered because they were Jews, beyond some scattered killings, as part of a systematic program of genocide. Maybe I'm reading too much (or not enough) into this, but I'm certainly not satisfied after reading what Gibson has to say about the Holocaust that he believes the Holocaust, as the word is commonly understood, occurred. And, while I'd like to give him the benefit of the doubt on this, and assume that he just used clumsy language, it's quite hard to do so knowing the views of his father (edite: which, because of the controversy surrounding them, suggest that Gibson would be both familiar with the views of Holocaust deniers and sensitive to the subtleties attendant to the controversy).
UPDATE: My correspondents are getting distracted by the side issue of whether Gibson should have raised gratuitously raised Soviet atrocities, etc., in the same context as the Holocaust (a tactic indeed used by Holocaust deniers and certainly "Holocaust minimizers", but hardly unique to them, and hardly good evidence of Holocaust denial, and I didn't claim it was!). Please focus on what I said was the main issue, the really troubling thing about what Gibson said: Holocaust deniers, at least the sophisticated ones, don't deny that Jews were sent to concentration camps, and don't deny that Jews suffered during the war, and perhaps suffered a bit disproportionately because they were Jews. What they do deny was that the Germans singled out the Jews for genocide, that millions of Jews were murdered, and that Jews were sent to death camps, not simply to labor or concentration camps. Nothing that Gibson said was inconsistent with the views of a Holocaust denier, and, indeed, as I pointed out, his statement sounds a lot like the stated views of the editor of the Holocaust-denying Journal of Historical Review. This all may be innocent on Gibson's part, and, if someone would ask him directly, "do you believe that the Germans murdered approximately six million Jews during World War II" and he said "yes" I would leave it at that. But given that he grew up in an anti-Semitic family, with a Holocaust-denying father, and has now asserted views that are very much consistent with the views of a Holocaust-denier, I can't say that my presumptions are with him at this point.
FURTHER UPDATE: I know some people begrudge all the attention that the Holocaust gets, given other tragedies of the 20th centuries that get a lot less attention. But that's a separate issue. Gibson has been accused of promoting anti-Semitism via his new movie. I haven't seen the movie, so I have no opinion on that. His father is a known Holocaust-denying anti-Semite. An interviewer asks, do you believe the Holocaust happened? Gibson doesn't just say, "yes, it did, of course." He doesn't even say, yes, of course, and we should remember it, along with other great tragedies of the 20th century. Instead he gives what appears to be an intentionally ambiguous answer that sounds an awful lot like the answers given by anti-Semitic Holocaust-deniers. Again, I'm not ready to condemn Gibson yet, just stating that his answer raises further questions.
FURTHER UPDATE: A reader suggests: "I agree with, mostly, with your Volokh post on this subject, but have you considered this explanation for Gibson's vague comment: he doesn't want to publically take a position diametrically opposed to his father's because he loves the old nut and has to live with him for many many more years? "