I was in the Nether trying to get the Camouflage and Return To Sender achievements as well as farm wither skulls for summoning the wither in the future. In over an hour of running around, I found: exactly 3 wither skeletons, 7 zombie pigmen, 0 endermen, 0 ghasts, 4 magma cubes, and one solitary regular skeleton. The only semi-plentiful mobs were blazes and only because of the spawners; only 4 were random spawns.

Also, blazes are not aggroing properly. I can walk right up and stare at them from a block away and they'll just look at me, then turn and float away. Only maybe one in two dozen gets pissed off and starts spitting fireballs like they're supposed to.