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 deviceís lawful uses.
The original case in Sony vs. Betamax is fundamentally different. The Betamax had a clear non-infringing use, it played Betamax tapes. The infringing uses only came about as consumers found out about taping TV shows to watch later. Quite frankly Grokster had no other use but copyright infringement and was marketed quite explicitly as such. Grokster's business model also relies on the exchange of copyrighted material as the display of ads was tied to the material. Clearly intent and marketing of services are what seperate the two decisions.
I think that pure protocal based approaches to P2P such as Bittorrent may pass the intent/inducement test. Bit Torrent is a very useful way to pass around large files, and has many substantial non infringing uses. (Updates are one example, ISOs are another.) For a complete history and player breakdown in this whole debate, News.com P2P coverage has pretty much every thing you want to know
