Eugene Volokh has written about how a slippery slop can actually occur in public policy decisions. He is now looking for examples of where 1. opponents of a new policy (say, anti-homosexual-discrimination laws) argue the law will lead to an undesirable outcome (say, same sex marriage) and 2. the proponents of a policy argue this will not happen, and 3. the feared outcome comes to pass (the Mass. Supreme Court decision mandating marriage equality for homosexuals).
His examples are discrimination laws that have supposedly led to same sex marriage, and Griswold v. Connecticut, which held that the State could not ban on the use of contraceptives for married couples, led to Lawrence v. Texas, which held that the State could not ban private, sexual, noncommercial conduct (gay sex).
I think there is a problem with his implicit argument that the earlier decision or law caused the later decision or law. While the precursor laws or court decisions are necessarily subsumed in the later law or court decision, they outcome is not necessarily caused by the earlier decision. Instead, the early law or decision and the later decision are the effects of social change and scientific understanding over time that move in a particular direction.
For example, Griswold v. Connecticut was decided in 1965. Over the next forty years, American society has become more sexually open, homosexuality is no longer identified as a disease, homosexuality is generally considered to be an innate trait rather than a lifestyle choice, women are expected to be treated equally in marital management and employment, etc. To claim that Lawrence was caused by Griswold simply because the Court cited Griswold, well, you must be a law professor.
He does have a point that Griswold made Lawrence more likely. So in hindsight, it looks like the Griswold opponents who argued a slippery slope are correct, and maybe Rick Santorum should not be mocked so much for his fear that incest and polygamy are next (although he should still be made fun of for thinking that man on dog sex might follow). However, the true power of the Griswold decision, or the decision to allow homosexuals to marry today, is that the fear-monger's predictions of the horrible things that will occur do not occur. People learn that their old taboo was not justified: Blacks should be able to marry whites; People should have a right to control when they wish to reproduce.
Thus, the people who argue the slippery slope are in effect saying, "if you pass a current law that most people think is reasonable, people will begin to realize I was wrong and start to disagree with me on other issues." While this may be true, I don't think it is a convincing argument. If I might change my mind in the future regarding incest because same sex marriage becomes legal, why should I fear my future, better informed self?