First, let's not have too many of these questions at any one time and certainly limit them during the private beta and the first few weeks afterward. We should avoid at all times questions like "How do you say #%$(@#)#@ in Spanish?" Beyond that, I think the standard for questions that are over the line should be like the standard for pornography in general: "I'll know it when I see it." Or to put it another way, we ought to be mature enough to recognize a legitimate question as opposed to a question asked to get some sort of rise.
To address Serabe's suggestionsuggestion, by all means warn the unsuspecting that they may be entering into dangerous waters with a particular word choice. But by it's nature, I doubt there would be many legitimate questions of this sort. Rather I would expect answers to point out vulgarities when appropriate.