Started this thread in 2009... finally had a game... hadn't realised the new site had gone live for free members....
The starting position is different. Pawns in the same place but the back row is randomized, with the same position on both sides. There are some rules for how they are actually placed and the the name comes from there being 960 valid starting configurations.
Castling is also slightly different, due to the changed positions, but the king and appropriate rook go to the same squares the would in a normal casting situation.
It is already there In chess.com
Look at the bottom of the page for a link that says "Try the new Chess.com". here you can play live chess960.
also in chess app
This is awesome. I waited 3 years for 960 to be added and now its here nobody will play me lol.