I have been meaning to write about this for some time but got sidetracked by all the other topics. It formed the basis of an article by Robert Rosenberg in the December 2005 issue of Physics Today (pages 50-55).
How people respond to such a question can tell you a lot about their relationship to science in general. Some people will think that such an everyday question should have a simple answer that has been known for a long time. Others will answer that that is just the way ice is. Intelligent design and other types of creationists might respond that ice was made slippery for a purpose that we cannot comprehend. Yet others will say “who cares?” and wonder why we should bother with trying to answer such a question at all. But ask a group of scientifically-minded people this question and you will get a lively discussion going about the various possible explanations.