Minecraft 14w20b, Minecraft 14w21a, Minecraft 14w21b, Minecraft 14w25a
Win 7 - 64bit
4 GB Ram
Java 7u55 64bit
Create normal superflat world. Creative mode, normal difficulty. Set it to day time to prevent other mobs from spawning.
See slimes spawning and despawning instantly in the distance. Very rarely, one will survive and won't despawn. But still, very rarely.
Note: I haven't tested with magma cubes, but I assume it's the same issue. On our snapshot server, I haven't seen a single one in 3 hours of gameplay.