Interesting coverage in Popular Science as well [1]. They point out (as does the newscientist article) that the key to organ printing is vascularization (creating a blood supply for the organ). Once you have that conquered you should be good to go on Kidney's, Liver's, Spleen's, Heart Tissue, Etc.

[1] http://www.popsci.com/science/article/2013-07/how-3-d-printi...