Maybe next to the OP's name? Sometimes extremely old threads get revived (how or why, I can't fathom), or a side conversation will pop up in a thread that was originally looking for restaurant recommendations for a vacation that was years ago. New posts will come up, and it's sometimes not until you've opened it or even responded that you realize it's totally moot. (By "you" I mean "I," of course.) I'd rather not have to open a post to see if someone still needs to know where to have lunch in Portland or if the lunch happened in 2007.