Unless you're talking the Nuvinci CVT(continuous variable transmission), tis a heavy beast, the earlier version being mainly used in electric carts etc:-
http://www.fallbrooktech.com/home.asp
The concept is really quite spiffy and they've got a lighter version out, the N360, with a 360% range but can be run with a double up front. The N360 weighs 2.5kg, which is a lot lighter than the earlier models, still heavy but I'd expect to see further improvements. I don't know the efficiency data for this hub but I suspect it's shy of the rest of the field at the moment.
If they get the efficiency up, the weight to 2kg(or less), the range above 400% and a reliability/strength factor akin to a Rohloff, then you're really talking a threat to deraillers.