In response to Fish, I'll weigh in on teaching of slang to Chinese students. How to make the decision what slang to teach and what not to teach? I realy have no boundries but do limit my involvement somewhat,e.g. if a student asks me about a slang expression I'll respond, or if it is contained within a text we are using, such as 21st Century Newspaper. I have often used slang expressions as a reference concerning the changing nature of language. My classes are usually free flowing with a central theme, "get them up and speaking in English" so sometimes I myself use slang to make a point or to show students how to say something differently. Also, sometimes to provide comic relief.

Recently, I had an experience teaching middle school English Teachers. (Chinese) One of my colleagues delivered a strong speech denouncing the teaching of slang, said she never used it, but when ask about teachers who advocate the teaching of slang, she said "they don't have a clue". Go figure! What say you Fish, what's your take on teaching slang. I think we can't deny slang is a intregral part of a language and it's OK to expose students to it, and help them understand it. After all, they do see it and hear it in Movies and Music. (dare I use the word Western?)
Any other non-confrontational discursive postings are certainly welcome!