From SCOTUSblog:

"The Supreme Court ruled unanimously that developers of software violate federal copyright law when they provide computer users with the means to share music and movie files downloaded from the Internet, at least when the software companies take "affirmative steps to foster  infringement."

In a decision announced by Justice David H. Souter, the Court said: "We hold 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, is liable for the resulting acts of infringement by third parties" — that is, computer users using free downloading software."

Other comments: ZDNet’s FAQ, Kim Weatherall on implications for Australia,  WSJ roundtable, Tech Law Advisor, Copyfight