Is it wrong only when you are saying negative things or is it equally wrong if you are talking about them with admiration or saying something nice?
Depends which side of the fence your on
I'd appreciate if anyone talked behind my back in admiration
I try not to say anything ABOUT someone that I would not say TO someone - it's a reasonable yard stick.
I can't think of a good reason why it would be wrong to say something nice about someone behind their back, though I think it would be preferable to say it to their face.
I'm with Janet on this one. If you can't tell them to their face, then don't tell anyone else!
Paul gives a rule for Christian speech that I like--say only those things which edify (which build people up--paraphrasing, say what is helpful).