When someone says, ""That's mighty white of you", what do they mean?

[aikoaiko]

I've had this said to me (but not in a long time) and I've heard this said of others, but I've never been really sure what it means.

From http://en.wikipedia.org/wiki/Play_the_white_man">Wiki: "That's mighty white of you",

From http://www.urbandictionary.com/define.php?term=mighty%20white%20of%20you">Urban Dictionary: Used to describe someone who thinks they've done a great deed, charitable action or sacrifice, but in reality they've done very little to help the human condition. Foolish phrase used by some to show so-called white superiority. Phrase attributed to the south, and coined during days of slavery. Now sometimes said in a sarcastic manner to people who act as they are superior because of their race. To behave or act in a manner stereotypically attributed to Caucasians

The former is not pejorative, but the UD definitions are. So what so you DU?