Our friend David Sobel, over at PEA Soup, has a very nice post discussing what he takes to be a serious problem with the Nozickian approach to property rights (one that probably applies to the Randian or Rothbardian approach as well).  The problem is that "such libertarian views cannot adequately distinguish the moral seriousness of the wide variety of kinds of actions that violate our property-rights. "  Read more here.  

Also, don't miss his earlier excellent post on libertarianism and risk.