Fred von Lohmann, Senior IP Attorney at the EFF, posted an interesting article on the current state of IP law in the US, which he has recently updated in the light of his victory in the Grokster case.
The MGM v. Grokster ruling suggests that, with careful attention to the relevant legal principles, indirect liability can be avoided by P2P developers. Because this case may still be appealed to the Supreme Court, however, developers should exercise caution in relying on the ruling.
You can read it here.