Server updated to Bukkit 1.6.4
This version is aiming to smooth the transition to 1.7's new world generation, particularly for generated structures (villages, witch huts, nether fortresses, strongholds, etc).
The problem is that the locations of some structures is based on the world's seed, and when updating to 1.7 these locations would move since world generation is changed so much. Basically will break nether fortresses and witch huts, and maybe some other things.
The solution is 1.6.4, which will save structure locations in our world independent of the world seed. However, this doesn't happen automatically, any structures we want to keep working properly, we'll have to visit now that 1.6.4 is up, so that the chunks get loaded, and the structure locations get saved.
TLDR: If you want your nether fortresses, witch huts, etc. to still work (spawn blazes, wither skeles, witches) you need to go on the server and visit that area sometime before we put 1.7 on the server. This won't be for a few weeks at least I imagine.