Of course, due to Aikar's optimizations, mobs further than 32 blocks will not move -- which makes slime farms much less effective. You can't be closer than 24 blocks or they won't spawn, but you can't be further than 32 or they won't move into the water streams that eventually break them down and drop them at your collection point.
There is actually a very complicated equation to discover if it is a slime chunk... but it only works if you have the seed of the world...
In vanilla the range is much longer, Aikar reduced it to 32 on EMC per this thread: http://empireminecraft.com/threads/emc-entity-changes-distance-to-activate.20728/
AI: All mobs except slimes will stop wandering within 5 seconds if there is no player within a 32 block radius. In this state, they will glance around randomly, but they won't walk anywhere. They can still be moved by other means such as flowing water, minecarts, etc. Zombies can walk when there is a villager they can attack even if they are over 32 blocks from a player, similarly villagers will flee when there is a zombie chasing them. http://www.minecraftwiki.net/wiki/Mobs
