I thought this was quite timely and informative since I had wondered about the precise definition myself.  An article in a San Diego paper delves into the background of the word “nappy-headed”:

Since slavery times, ?nappy? has been used to malign the natural hair texture of many people of African descent: dense, dark and tightly curled. So when Don Imus referred to the women of the Rutgers basketball team as ?nappy-headed hos? ? a widely condemned remark that got him fired Thursday ? it cut deeper than many who are unfamiliar with the term might realize.

http://www.signonsandiego.com/news/nation/20070412-1500-nappyhair.htmlBut also interesting about the story was this part:

The irony of Imus' comment is that many of the Rutgers players have straightened hair ? and at their news conference Wednesday, it seemed nary a straightened hair was out of place.

Doesn't this mean that Imus evidently didn't know what “nappy-headed” means?  If so, then his comment had nothing to do with the texture of hair of African Americans…