Aristotle thought that some people were naturally slaves, and it's hard to overestimate the historical importance of Aristotle on philosophy. See http://www.suite101.com/content/aristotle-on-slavery--some-people-are-slaves-by-nature-a252374
More recently, George Orwell wrote about the waiters in the Parisian restaurant where he was a dishwasher:
...never be sorry for a waiter. Sometimes when you sit in a restaurant, still stuffing yourself half an hour after closing time, you feel that the tired waiter at your side must surely be despising you. But he is not. He is not thinking as he looks at you, ‘What an overfed lout’; he is thinking, ‘One day, when I have saved enough money, I shall be able to imitate that man.’ He is ministering to a kind of pleasure he thoroughly understands and admires. And that is why waiters are seldom Socialists, have no effective trade union, and will work twelve hours a day — they work fifteen hours, seven days a week, in many cafes. They are snobs, and they find the servile nature of their work rather congenial.
See http://ebooks.adelaide.edu.au/o/orwell/george/o79d/chapter14.html
Lots of people get real pleasure out of pleasing authority. Not out of being abused by authority, but rather by getting little pats on the head. Elves are not far from human at all in that way.
Dogs love to be praised by master, and that's why we love them. Dogs were bred by humans from wolves to be obedient and submissive, if not sentient.
EDIT/P.S. Slightly off-topic. Epictetus, one of the founders of stoic philosophy, was himself a slave for much of his life. His thought was absolutely a rationalist. As far as I know, he never questioned slavery as an institution.
It may not be exactly what Aristotle had in mind, but I feel obliged to point out that some people do consider themselves to be natural slaves, and I would consider it rude to contradict them about that. If you want one they can be obtained on collarme.com, Or So I've Heard.
Edit: This is old material. It may be out of date.
I'm talking about the fictional race of House Elves from the Harry Potter universe first written about by J. K. Rowling and then uplifted in a grand act of fan-fiction by Elizer Yudkowsky. Unless severely mistreated they enjoy servitude to their masters (or more accurately the current residents of the homes they are binded to), this is also enforced by magical means since they must follow the letter if not the spirit of their master's direct order.
Overall treating House Elves the way they would like to be treated appears more or less sensible and don't feel like debating this if people don't disagree. Changing agents without their consent or knowledge seems obviously wrong, so turning someone into a servant creatures seem intuitively wrong. I can also understand that many people would mind their descendants being modified in such a fashion, perhaps their dis-utility is enough to offset the utility of their modified descendants. However how true is this of distant descendants that only share passing resemblance? I think a helpful reminder of scale might be our own self domestication.
Assuming one created elf like creatures ex nihilo, not as slightly modified versions of a existing species why would one not want to bring a mind into existence that would value its own existence and benefits you, as long as the act of creation or their existence in itself does not represents huge enough dis-utility? This seems somewhat related to the argument Robin Hanson once made that any creatures that can pay for their own existance and would value their own existance should be created.
I didn't mention this in the many HP fan fiction threads because I want a more general debate on the treatment and creation of such a class of agents.
Edit: Clearly if the species or class contains exceptions there should be ways for them to pursue their differing values.