There is currently no official release date set. Nintendo has only said sometime in 2012. Many online publications are speculating summer of 2012.
The game will be rentable on release day. In fact, GameFly generally sends out new releases 1-2 days before release day so members can receive the games as close to release day as possible.
You may add the game to your GameQ now. I suggest doing so if you want to have any chance of getting from GameFly on or around release day.