I first found out about this here:

http://www.youtube.com/watch?v=22ZofcpevtY&t=5m42s

I didn't know the NPCs were still spawning! I'm an achievement junkie, and I'm DYING to get this achiev, since it obviously can only be earned for a limited time.

Please post any specifics that you know (time of spawns, areas of town or camps)!