When do stand up comedians cross the line?

This question is inspired by the articles floating around about an anti-gay tirade that was (purportedly) launched by Tracy Morgan in his set last week. So far I have come across accounts by audience members, the theater has denied association with Morgan and his publicized has said “no comment” (Source.) I don’t know what was actually said that night, whether this is all part of Morgan’s elaborate, offensive personae, but I don’t think that really matters.

What I want to know is whether comedians in general cross the line. Are there certain topics that shouldn’t be touched? Does it have to do with how they say the things they say? Is it okay to say things that may be offensive if it is in jest rather than pure hate speech? What’s the difference?

I am very anti-censorship but it’s shit like this that makes my insides churn.