A key question on the ethics of it all is - I think - whether there is some externality from the behavior that leads to social utility. Reading literature might make you more understanding and empathetic of others' experiences for example. Gambling in a casino, maybe not so much (though certainly it creates jobs, etc).

I'm excited by the idea that neurochemical feedback loops in gaming could create some positive social utility. Certainly the storytelling in a lot of games is artistic and meaningful, but maybe we'll find even better ways to directly channel those brain cycles, along the lines of protein folding games but with more depth. (Neal Stephenson had some interesting ideas for this in Fall or Reamde - can't remember - where an MMORPG had an API similar to mechanical turk where players were rewarded for cognitive tasks in the real world like airport security)

edit: I guess we shouldn't forget that games are fun too, and doesn't necessarily need to be deeper than that to be good.