Wither Skeleton and Blazes do not spawn in pre 1.4 Nether Fortresses and seem to only spawn in most newly loaded Fortresses. ex: playing 1.4.2 and discover Nether Fortress, Wither Skeletons and Blazes spawn naturally, and when discovering a new fortress Wither skeletons and Blazes spawn naturally, but upon revisiting prior fortress Wither skeletons and Blazes do not apear to spawn natually.