Here's a nice article at the North County Times commenting on the recent death of Nadi at San Diego Zoo, and what the future holds for the Northern White Rhino. There's also a video report (poor sound quality unfortunately) where you can see the zoo's remaining Northern White Rhinos up close.

Only a handful of northern white rhinos remain, including the Wild Animal Park's elderly female, Nola, and a male named Angalifu.

"Things look incredibly dismal at this point," said Randy Rieches, curator of mammals at the Wild Animal Park. "The only hope may be to preserve as much genetic material as possible and wait until reproductive technology advances enough to bring the species back."
[...]
A 2006 news statement from the foundation says:

"Although the African Parks Foundation is reluctant to give away information on the remaining few (rhinos), they have always been living safely away from the clutches of the LRA. Rather, their main threat has been raiding parties of well-armed Sudanese poachers."
Randy Rieches' comment in the middle of the article bears repeating here... A great quote!
There's a big difference between a species that goes extinct naturally and one that goes extinct because of human greed.