We shouldn't put too much faith in the established version of history. As someone noted above, history does tend to be written by (or to please) the winners. And no historian is completely without bias, conscious or otherwise. Even eyewitness testimony is necessarily suspect. People lie; they see things from different perspectives; their stories are colored by preconceived assumptions. And people forget. (One of the key witnesses to JFK's assassination recalled waving to Kennedy just as he was shot, then a moment after the shooting running across the street and almost getting hit by a motorcycle. She almost certainly THOUGHT she was telling the truth, but the Zapruder film shows her standing completely motionless throughout the entire thing.) When we look at the distant past, there's often very little evidence to go on. When we look at the recent past, there's TOO MUCH evidence -- no one will ever have time to sift through all the paperwork generated by the Clinton Administration.
Of course we can say with reasonable certainty that Columbus visited the New World, but he may have been preceded not only by the Vikings but by English fishermen in his own period. And did he REALLY believe he was sailing to the East Indies, or was he fully aware there was another landmass between Europe and Asia, and just using the Indies cover story because it was more saleable?
There is no serious doubt that the Holocaust happened, but there is (and probably always will be) uncertainty about the numbers and other details. I think it's incredibly counterproductive, however, to declare any questioning of the Holocaust forbidden. It almost suggests that there is something to hide, that the official version can't stand up to scrutiny. You can understand the impulse to enshrine the memory -- especially at the time, when the enormity and heinousness of the crime truly boggled the mind, inevitably producing the initial reaction, "It can't be!" Unfortunately, though, we now know all too well that it is possible to murder thousands and even millions of people on account of their ethnicity or religion, and in a surprisingly short period of time. The Holocaust has become all too conceivable an event.
I do understand the importance of keeping the memory of the Holocaust alive. But historians should approach it with the same critical, questioning instinct they apply to the rest of the past. It should not be a sensitive topic, glossed over on the tour, that the gas chamber at Auschwitz is largely a restoration. Indeed, because this is apparently rarely mentioned, deniers trumpet it as evidence that the entire Holocaust is a fabrication.