Did some searching here and on the wiki and didn't see this mentioned yet. The latest snapshot adds a jungle biome which is pretty cool. Unfortunately, you'll never see one in any of your old worlds, you have to create a new world, so says Mojang.
Not so. I just converted a 1.1 world over to the new snapshot format and got a jungle. The thing is, and this is what Mojang is probably trying to avoid, you get chunk errors, of course. It's great not having chunk errors, but I would trade them for the new content in my old world any day. Plus you can always make them not quite so ugly with a tool like MC Merge http://www.minecraftforum.net/topic/629884-a-tool-for-merging-17181910-maps-mcmerge-v052/
Anyway, an entry has been added to the world save dat file that defines what terrain generator to use.
Back up your world save folder in case something goes wrong. Don't say I didn't warn you.
Open your map once in the newest snapshot. This will be the official version soon, I would expect. Unless something changes then, it should be the same.
Run NBT Edit and open the main dat file for your world. Find it in the world save folder, it's called level.dat.
Just for comparison, here's an old (alpha) world in NBT Edit.
Note there's only a generatorName, called DEFAULT.
Now, once the world has been opened once in the latest version, we see a new data field has been added, called generatorVersion:
With a value of zero, and generatorName is now default_1_1.
Here's a world that was created in 1.1 but has been changed over to include jungle biomes.
Pretty simple, change generatorVersion to 1 and generatorName to default, save the file, run minecraft, and go looking for jungles. But expect chunk alignment errors, the severity of which will depend on the original terrain version of the map.
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