Dissent

"While the Court unanimously concurred that Grokster could be liable for inducing copyright infringement, there was considerable disagreement over whether the case is substantially different from the Sony case, and whether the precedent established by Sony should be modified. On one hand, Justice Ginsburg, joined by Kennedy and Rehnquist, claim that "[t]his case differs markedly from Sony" based on insufficient evidence of noninfringing uses. On the other hand, Justice Breyer, joined by Stevens and O'Connor, claims "a strong demonstrated need for modifying Sony (or for interpreting Sony's standard more strictly) has not yet been shown," primarily because "the nature of [...] lawfully swapped files is such that it is reasonable to infer quantities of current lawful use roughly approximate to those at issue in Sony." These justices concur in the judgment on the narrow ground of Grokster's alleged inducement of its customers to use the product illegally."
http://en.wikipedia.org/wiki/MGM_Studios,_Inc._v._Grokster,_Ltd.