My feeling is that Dwarves would probably trap and enslave a few brute creatures like trolls for use as heavy labor in the depths. I doubt they'd bother to enslave humans or elves, though... trolls would just make
better slaves overall, being both stupider and stronger. Plus, since thery're less "civilized", the dwarves could (like many slaveowners in the real world) tell themselves that they were doing the savage trolls a favor. They might not call them slaves, calling them captured laborers or something like that (which might help squeamish players), but the end result is the same.
I don't think kobolds or goblins would make good slaves. They're scrawny, sneaky, clever, and quick to backstab. They're probably good at things like picking locks, too... as slaves, they'd just be more trouble then they're worth. Most things could be done better by a minimum-wage Dwarf, and the things that couldn't be are things you probably don't want a slave doing.
I don't think Dwarves would enslave other dwarves. They seem better than humans that way.
Most other races are no good at surviving in the caves, making them useless as slaves. Kinda pointless to enslave a human or an elf when they'll just die from the underground air, poor nutrition, and so forth anyway. Plus, the humans, at least, can't see very well in the dark.
Regarding the other races keeping slaves... I'd imagine some of the monstrous humanoids would keep other nasty humanoids as slaves (maybe goblins have some kobold slaves, or vice-versa). Everyone would probably want a few trolls and other big stupid nasties if they could manage to keep them. Elves would probably not want to even interact with the savage (ie non-elven) races that frequently... they might keep a few creatures to try and 'reform' them, though. Dwarves are probably too stubborn to make good slaves for anyone else, at least when other options are available. Probably Elves are the same way.
Humans... well, we already know about humans. Humans are horrible. They're really the nastiest monster race in fantasy, when you think about it. Humans enslave other humans. How horrible is that? Goblins probably tell each other horror stories about the monstrous inhumanity of humans to scare themselves to sleep.
[ August 25, 2007: Message edited by: Aquillion ]