When in a world and you change to peaceful, mobs in the distance remain alive. If you get near them they will despawn.

Steps to recreate:
1. This works best in a flatland.
2. Set the difficulty to normal.
3. Set the time to night.
4. Turn to peaceful.
5. Look around, you should see mobs remaining.
6. Type "/say @e". It should list a bunch of mobs that shouldn't be there..

You can always type "/kill @e[type=!Player]" to get rid of these mobs, but that is a waste of time.

NOTE: (this has been tested in 1.8.1 only)
When using a large render distance such as 32, it seems to get rid of the mobs.

MY THOUGHTS ON THIS:
I fully understand why Mojang has ignored this. If this were to be fixed, then Minecraft would have to have every single chunk loaded constantly in the world in order to do this. It would be quite hard to find an alternative.

THE OTHER OPTION:
Of course, you could make mobs run on a different thread then the rest of a game, but this would probably be quite laggy in the long run. I personally don't support this being fixed at the moment, and nor do I think this urgent. There would have to be a complete rehaul on the Minecraft mob system and I don't think it's worth it.