Accoring to many tests, animals should not respawn. However, it's have not been the case lately on my server. Reports about missing animals from farms etc. came in after 1.0.1, i thought that there where a little animal killer going around. Reports keeps coming in, and i did dom research in the log (logblock). The result is: either the animals are suiciding on my server, or there is a bug in bukkit or one of my plugins. Plugins: Code: GroupManager MagicCarpet CombatTag OnlineUsers Permissions WorldEdit Multiverse-Core LogBlockQuestioner Essentials LagMeter EssentialsSpawn EssentialsChat WorldBorder WorldGuard Multiverse-NetherPortals Multiverse-Portals AntiBuild EssentialsGeoIP LWC Train Carts LogBlock Showcase dynmap Bukkit 1.0.1-R1 and all plugins updated to latest version, except the 3 multiverse (a few days old dev version).
I have a similar setup, and my despawning animals were loading into the game inside a wall or on the wrong side of fences. It seems that minecraft plays fast and lose with their respawn locations, so the combination of the two factors I noticed leads to suspicious disappearances.