Due to the fact that mobs despawn when far from other players, you cannot move hostile mobs through Nether portals in SMP if there is a player online on the other side of that portal, unless that player is close enough that the mob does not despawn. Everything works fine in SSP, or if there are no other players online, or if it's a mob type that doesn't despawn (i.e. passive mobs)

What I expected to happen was...:
The hostile mob should move through the Nether portal but then wait "in limbo" on the other side, because the chunk in which the portal they'll come out of isn't loaded (which is how it works in single player)

What actually happened was...:
A mob will move through the portal on one side, arrive on the other side, see that it is too far from the other players (even though the chunk isn't loaded) and despawn instantly

Steps to Reproduce:
1. Start a SMP server
2. Have two people online, both in the Overworld, and very far apart (thousands of blocks, whatever)
3. Have one person create a Nether portal, go to the Nether, and move a hostile mob through the Nether portal back into the Overworld
4. The mob will despawn

Other tests that confirm this behaviour:

• If the "other online person" is close to the Nether portal, the mob moves through successfully
• If no one is online at all (or alternatively, in single player), the mob "waits in limbo" and loads successfully when the player moves through the portal
• Passive mobs move through and do not despawn, regardless of game mode or who is online
• Despawning works both ways (Overworld > Nether or vice-versa), so long as there is a player out of range on whatever side of the portal the hostile mob comes out of (in other words, if player 2 is in the Overworld and no one is in the Nether, and you move a hostile mob to the Nether instead of from it, the mob will be waiting for player 1 in the Nether