I am having trouble getting any slimes to spawn in SMP. I have dug out 2 slime chunks with the floor at y=6.6, lit it up, and left it there for about 2 hours.
Getting the world files, stripping all entities, and plugging them into SSP got me slimes almost immediately, in those chunks. Copying the world back to the server didn't make a difference.
On a related note, it seems that SSP will spawn way more mobs than SMP - I got pummeled by ~10 creepers in SSP, while in SMP, running into hostile mobs is a rare event.
Does this have to do anything with the server settings (I tested on Normal in SSP and hard in SMP), or did Mojang botch something up?
Proof this is a slime chunk: