Suppose action A is morally wrong. Person S does not plan to do A because s/he sincerely believes that doing it is wrong. However, s/he is very tempted to do A, and so S constantly thinks about doing A but never actually does A. Is S morally culpable?
Aristotle comes very close to answering this by saying that the unrestrained person is morally culpable because he knows that A is wrong, but nevertheless does it. However, my example above never says that S actually does A; S wants to do A and desires to do A but never actually does A. Can thinking–but not doing–an immoral action still be morally wrong?
Shaun Miller's Ideas 
I don’t think this one is so hard to answer, because in the example you’ve given there aren’t any negative action consequences from thinking about A. We could imagine a scenario where the negative thoughts lead to negative actions more reliably.
For example, let’s say that S thinks, regularly and without attempting to change them, derogatory racist thoughts about a particular race. A consequentialist ethic could blame these thoughts for what must be an almost inevitable failure to show as much consideration towards people of that race exhibited in S’ actions; the link between thought and action feels much more reliable to us there.
So, this is probably the obvious answer from a consequentialist reader — the thought is morally wrong to the extent that it reliably leads to actual negative consequences. (Virtue ethics would have a different answer, I’m sure, although you can encompass that answer into consequentialism by broadening the horizon of how far ahead the consequences are enough.)
The results are very different between acting and thinking.
Maybe it matters ‘why’ S didn’t act. If S wants to shoot someone but can’t act because there is no gun, versus not acting because s knows the action is wrong.
This is an interesting case. We often say that there’s a difference between acting and thinking, but in the real world, I think we sometimes equate them. Let me use Vanessa’s example. Suppose your neighbor is cordial and friendly to you and you suspect nothing could be wrong. Suppose your other neighbor is S and S wants to shoot someone but doesn’t do so because S believes it would be wrong. In all seriousness, would you want this person living next door to you? Even though this person hasn’t done anything, nor plans to, it still seems that we judge S to be morally wrong, not in virtue of S’s actions, but of S’s character.
I think Vanessa’s response is best. The “why” is ultimately important.
Shaun, I get what you’re saying about still judging person S to be morally wrong. Weird how we do that.