I have a feeling that given the magnitude of this merger the actual 'end' of NW will be a bit fuzzy for some time. A lot of NW planes have DL external livery and the interiors are slowly migrating but some things take a long time to die. If you look at some planes you can still find Republic markings for instance.
For flyertalkers I guess the 'good bye' will be the end of Sep DO - see the sticky at the top of the forum for the pointer.