Fergal has a very readable essay on the MGM v Grokster on Tuppenceworth.ie today. He explains the kinds of things that are being contested in the US Supreme Court case, and gives a bit of historical background to the previous legal decisions in the same area. At stake, essentially, is the question of whether inventors can be held responsible if the useful gizmo they come up with (photocopier, VCR, iPod, BitTorrent, biro) turns out also to make it easier to breach copyright.
And therefore whether we ever get to use any of these potential future gizmos legally.