According to SCOTUSblog,
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.”
“With the object of promoting its use to infringe copyright.” Not exactly a bright line test.