Monster dungeons (Spawners) are not spawning on new worlds.
What I expected to happen was...:
I re-created a previous world (Created on previous snapshot) and dug to where I knew there were 2 dungeons side by side. They were not there. Upon further inspection, I did not see any monster dungeons at all.
I have attached 2 pics from Minutor showing the first map, and the re-created 14w04a map
Towns.png was created on snapshot 14w03b
Townies.png was created on snapshot 14w04a
I included a large area so you could see they were in fact the same map, and same depth.