There was a lot of talk about this subject at GDC this year. Lots of designers feel really uncomfortable with the Skinner box overtones of some of the facebook games and constant reinforcement that's become a standard part of the design toolkit. If the only thing you're offering is a slot machine or Skinner box without something else, then I personally think ethical questions start becoming important. But I also think that you've moved beyond the realm of designing and developing games and into the realm of gambling systems.
Addictive games that are addictive because they're compelling products that people have fun with? I find it hard to have any ethical qualms about making those. I have been a victim of Civilization's "just one more turn" effect since the early nineties, but I don't think there's any ethical stain on Sid Meier, Bryan Reynolds, Soren Johnson, or any of their teams. Sure, parts of these games could be viewed as psychologically manipulative, but then again, so could changing elements of a website according to psychological principles to increase conversion. The ethics and morality of using these tools hinges on intent.