Since Build #1 and through all Builds up to and including the latest Build #8 I have noticed that Hostile Mobs do not ever despawn, no matter how far away you go, or for how long, it is difficult to single out a particular mob to test this but using the prodedure described below I believe provides sufficient evidence for this conjecture.

How to recreate: Dig a pit 8x8 wide by 4 blocks deep, wait for hostile mobs to either spawn or fall into the pit you have dug (remembering not to interact with them), once you have a hostile mob or mobs within the pit, use crouch/sneak to place sufficient torches within the pit to negate any further mobs spawning within, cover the pit with a different block (glass is best for viewing purposes), then travel several thousand blocks away from the pit, wait there for as many games days or weeks as you see fit, then return to find the exact same mobs waiting for you in the pit, thus proving that the mobs do not in fact despawn.

Hopefully this is clearly worded and entails a minimum amount of ambiguity.