Wasn't this question answered in "When Harry Met Sally"? The real question is why?
Dr. John Grohol over at PsychCentral published this article yesterday: Why Women (And Men!) Fake Orgasm? The results of a study done at the University of Kansas (my alma mater) aren't exactly surprising:
Read the article. And then enjoy the video. It's a hoot.
The older woman at the end of the clip is the director, Rob Reiner's mother, Carl Reiner's wife, Estelle. She practically steals the scene in less than two seconds.