What I expected to happen was...:
The Mobs would not allow themselves to fall into the void.
What Actually Happened was:
The mobs Fell into the void. (watch the whole video)
http://www.youtube.com/watch?v=77U7k1tRjkE&feature=youtu.be
Steps to Reproduce:
1. Dig down near the void, make sure the void is accessable.
2. Spawn a mob on a block
3. Wait for it to move, or make it move (without pushing) into the void.
It happens because the Mob AI looks for air blocks, But there are no air blocks in the void.
DUPLICATE OF CLOSED ISSUE MC-3151, The issue was labeled as "invalid" I now have proof that it is not invalid.
Linked issues
Comments
Bump, no response.

Possible fix
Using MCP naming.
PathFinder.getSafePoint()
...
if (var9++ >= entity.func_82143_as()) {
return null;
}
--y;
if (y > 0) {
var7 = this.openPoint(x, y, z); // Accept a point in mid air, eh?
} else { // FIX: lets cancel that possible mid air point if reached void.
return null; // FIX
} // FIX
...
Tested on 1.4.7 and at least creepers learned to look down.

Affects 13w09b.

Is this still a concern in the latest Minecraft version 14w08a? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.
Still an issue in 1.7.4 and 1.8 snapshots

Resolved back to MC-3151, since that ticket was reopened.
Fix by Markku attached there.
This is very annoying, I'm trying to make a survival island map, and mobs keep falling into the void!
I guess I hust have to use a map editor to adjust the y-coords of my island...