Currently (2010-04-01) it's still in open Beta format, and has yet to be fully released. The beta is open to all who have bought the full version of FC.
Fantastic Contraption 2 adds magnets (positive and negative) to the supply of things you can build, and also features new things for the level design: moving/rotating floors and pinned nodes. Contraptions can now be pre-set on the level design, and the interface is updated. Upon completion of a level, the game now keeps track of the "badges" (achievments) you have earned, for example if you have used no powered wheels, no rods, if the goal piece stays in the goal forever, and so on. The interface for the level creation is also vastly updated.