This is a good question. Strangely, though, the answer is yes! I once got penalised on a university paper for doing just that - I used a section from a previous assignment that was also relevant for the current one. Although I protested, the logic was clear in that I had plagiarized existing content and tried to pass it off as new content.

However, unlike university, there's no real rule against doing that here. As a self-policing community we ought to vote with our feet on this one. Personally I don't see anything wrong with reposting old content, as long as the privilege is not abused by reshuffling old posts on a weekly basis at the expense of creating new content and adding further value to the network. As a kind of honour system, it would also be good if people made it clear when they're reposting, either in the title or perhaps as a new tag.