Samsung has been mass-producing 3D NAND modules since August last year according to this source. Here is a most informative news on Samsung 3D-VNAND technology from iTers News discussing limits to 2D scaling beyond 10 nm and "3D scaling".
Here is a interesting presentation from Applied Materials on 3D NAND "Winning in 3D NAND" explaining quite some details on process requirements for going from 2D to 3D technology.
"Advantage of 3D NAND is that it doesn’t require leading-edge lithography…the burden will shift from lithography to deposition and etch." Ritu Shrivastava, VP Technology, SanDisk, May 2013 (SemiMD) [Picture is a screenshot from Applied Materials presentation linked above]