In the Grokster case, the Supreme Court unanimously decided that "One who distributes a device with the object of promoting its use to infringe copyright, as shown by clear expression or other affirmative steps taken to foster infringement, going beyond mere distribution with knowledge of third-party action, is liable for the resulting acts of infringement by third parties using the device, regardless of the device's lawful uses."
And they remanded the case (see Michael D. Bryan's analysis) "saying that there is room to find imputed liability for copyright infringement by Groksters users despite the Betamax case because Grokster solicited users to use their product specifically to violate copyrights."
The Court did not say which actions short of marketing/advertising your service for the purpose of violating copyright would confer liability, so the essence of file-sharing — providing the technology to enable it — was left an open question.
Perspective on the decision seems directly related to economic stake in the case. Here is Neil Portnow, president of the NARAS on the ruling:
The National Academy of Recording Arts & Sciences represents 17,000 musicians, composers, artists, engineers, producers and songwriters and is dedicated to improving the quality of life and cultural condition for music and its makers.
Today is a good day for music fans and the 17,000 musicians, composers, artists, engineers, producers and songwriters that are the members of the Recording Academy. By unanimously upholding the rights of creators, the Supreme Court has defended an environment for legal online music services to thrive.
As the National Academy of Recording Arts AND Sciences, our membership embraces new technologies that deliver their music to fans in innovative ways. The court is forging the way for the legal digital services - those that compensate the creative professionals - to enable music fans to hear their favorite artists wherever, whenever and however they want.
The opinion also sends a strong message in defense of copyright protection, defending artists' rights to create and to profit from their works within a safe and supportive system, while also making sure those rights are taken into consideration as technology continues to evolve.