I have built a mob trap around y 90 (as can be viewed in the attached image), which does not spawn mobs even when the surrounding caves are lit, you are 25 blocks away, and the server is cycled between PEACEFUL and NORMAL modes to ensure mobs can spawn.

I imported the entire thing into creative using MCEdit, and determined when x and z are positive, the trap works perfectly. When x and z are negative, it does not work at all. When one is positive and the other negative, some orientations of the trap work, and not others.

This has been in minecraft since at least 1.2

I've attached the save where the screenshot is from, and you can find the trap at the coordinates from the image.