I can't find a specific reference now, but I'm sure I've always read that you want to build mob farms 24-32 blocks from where you wait, because mobs will not spawn closer than 24 blocks from you, and further than 32, they will despawn after a few minutes. From what I had seen in the past, this was true: standing just outside the spawn area in a mob farm would stop them from spawning. With the recent snapshots however, I have had mobs spawn directly in front of me ( 1-2 blocks distance ) while roaming around outside at night and instantly hit me.