I just moved my server to new hardware and updated to build 2430 of CraftBukkit, and I've noticed slimes spawning on the surface. Is this a problem with Bukkit, or has my server's world gotten screwed up on the move to the new computer?
If it's a flat map, slimes always spawn on flat maps. You need to get a mob control plugin. And slimes now spawn in swamps in regular worlds. I hope this helped.