Nether mobs are not spawning in about a 16 block radius sphere around 0, 64, 0.
I've encountered this in two worlds. The seed to one of which is -3177630428171215745. After hitting this a second time I spent some time checking the area they won't spawn and found it to be a perfect sphere.
Since I can't imagine the use of such a dead zone I figured I would report it as a bug.
I was going to do some additional testing on this tonight, but, am unable to replicate it in 1.8.2-pre7