A woman yells it in the middle of a Palin speech (around 0:38). Palin halts - and clearly heard it. Depressing. (Update: I haven't independently verified this video. As a reader reminds me, these things can be doctored. Watch with skepticism. If I get any more info, I'll post.)
This article available online at:
http://www.theatlantic.com/daily-dish/archive/2008/10/the-n-word/209538/
