Last I heard about him is that he and Arthas merged to become the LK and I don't think I have heard his name ever mention since it was either the Lich King or Arthas. Or am I missing something about Ner'zhul? O.o Also the LK doesn't sound like two people if you compare it to the original.
http://www.youtube.com/watch?v=QlubNJQbxgg