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To answer your question, it is legal if you do it right. There is a Meta.stackoverflow question that goes into some of the details, and gives a list of sites that do such:

http://meta.stackoverflow.com/questions/24611/is-it-legal-to-copy-stack-overflow-questions-and-answers

That's an edge-case, though likely infringing. I'll poke some of the SE people who check into these things.