I'm surprised that people don't make the obvious point about holocaust denial: that it is motivated by an ideological ulterior motive so obvious that calling it hidden agenda seems foolish. The motive is the rehabilitation of the Nazi project, in the largest sense: I don't mean the posturing of 'neo-Nazis', I mean the assimilation of Nazism into the political mainstream. What is particularly significant is the increasingly general sense nowadays that only the holocaust stands between us and an ability to admire 'what the Nazis achieved'. Take the holocaust away, and the road is clear not only to a reassessment of the economic, social and military successes of Nazism, but (crucially) to a reinstallation of these perennially popular ideologies at the heart of contemporary politics. In other words, at the core of holocaust denial is not so much the refusal of the fact of the holocaust, as the belief that mass-murdering Jews was an 'eccentric' or 'non-central' feature of what Nazism was 'really' about. Holocaust denial is in fact a denial that this malign sun was at the very heart of the Nazi regime, and a contrary insistence upon a kind of extreme geocentrism in which Nazi evil orbits somewhere outside Pluto (for not even the most dyed-in-the-wool holocaust denier would deny that the Nazis killed some Jews...). To put it another way: what motivates holocause denial is a peculiar sort of intellectual regret. The regret is that Hitler 'spoiled' his splendid achievement with this 'atypical' hostility against the Jews; if he hadn't done so, then his ideas would achieve the general purchase they deserve. That, rather than the fatuous factual denial, is the most poisonous feature of the phenomenon.
This is idelogically strategic in several ways. One is that other Nazi crimes against humanity fade from contemporary culture; that Hitler becomes perceived as 'just another' ... let's say 'just another Napoleon' or 'just another Frederick the Great'. Another is the way the holocaust becomes a problem not in an intrinsic sense but in an interpretive one: something to be 'dealt with' in one way or another.
This, of course, is not to contradict but rather to reaffirm what Deborah Lipstadt says in her Denying the Holocaust -- The Growing Assault onTruth and Memory: "the central assertion for the deniers is that Jews are not victims but victimizers. They 'stole' billions in reparations, destroyed Germany's good name by spreading the 'myth' of the Holocaust, and won international sympathy because of what they claimed had been done to them. In the paramount miscarriage of injustice, they used the world's sympathy to 'displace' another people so that the state of Israel could be established."