After generating a world in snapshot 13w09c, and later exploring the world more in the 1.5 pre-release, an emerald ore was found in a swamp biome, about 3 blocks from an extreme hills edge (We don't believe that chunk was loaded before the pre-release). I am unsure that this is actually a bug, or if emeralds can generate slightly out of extreme hills biomes.

A video of this can be found here: http://youtu.be/6LlnUTeUbx4 at roughly 15:30

Seed: -2434984148107909905
Coords: -228,23,198