I think that the double standard is as much from women as from men. I know way more women that talk shit about other women, than men. Hardly ever do I hear a guy say that some woman is a slut, or if he does it doesn't really mean anything very negative, just that she likes to have sex. However, when a woman calls another woman a slut it's usually much more hateful than when a man does it and the women means to slander the person she is talking about.
This could in part be the crowds I run in, I don't hang out with fundy types and most of my friends tend to be fairly progressive about sex (they have no problem with gays, they don't mind sexually aggressive women, etc).