@jwer: Let's take a look how many Northern White Rhinos exist:
-probably ~4 in the wild
-2 (1,1) in SDWP, one not fertile, one not behaviorally receptive
-Dvůr Králové Zoo: 7 (3,4) "pure" and one hybrid; as far as I remember, only one of the males and two of the females are considered for mating

All in all a pretty small founding base, even if additional animals from Uganda are brought in.

About possible reintroduction problems: one would be the resistance of the Southern Whites against certain diseases there that do not occur in their native range. One example could be Nagana, a protozoan disease (especially Trypanosoma congolense as well as other trypanosomes) transmitted by tsetse-flies (Glossina sp.). Most populations of Southern whites occur below the "tsetse-fly belt" (i.e. the habitat of tsetse flies in Africa), while the Northern whites live in this very area. Northern's seem to cope with trypanosomasis and might even be parasite carriers without negative consequences for the animal; whether this is also true for the Southern whites-?
For comparison, a map of the distribution of the white rhinos (orange = Northern, green = Southern)
http://upload.wikimedia.org/wikipedi...hite_rhino.png
and the distribution of tsetse flies:
http://www.au-ibar.org/isctrc/26Meet...s/image018.jpg

Even if SF movies want to make You otherwise- it's not "just": put the "genes" in the freezer, then later on put them into the microwave-and presto: You've got a Northern White Rhino! Trust me, it's not that easy. Instead, one should rather use now what is available.