If you update and then go to the edge of the map, it should begin to generate 1.2 biomes from that point on.
The update creates this funky looking boundary where the world generated by the previous version is cut off abruptly and the updated generator picks up. I had that happen on my now-defunct server, there was a mountain range that got sliced in half and left these giant rock walls that my friends and I used for pixel art with wool.
That's what happened on my previous updates on the server or my single player maps. 1.7.3 -> 1.8.1 -> 1.0.0 all did this.
However, the snapshot given to us specifically stated that you need to create a new map to get the jungle biomes to appear (They are indeed fantastic, checked them out a while ago, my favourite biome now. ). I don't know how it's going to work with this one. We'll see.