Edit: Sweet. We actually have a Bump Topic button. I've never noticed that before.
That is *wicked cool*. Ooooh, I'm so going to bump a few old threads...
EDIT: seems like you need: a thread in which you have the last post and that has some other property. Possibly it not being in top position or something. Or perhaps latest post older than a certain time. Then again, you could probably find it in the documentation or in the source. Don't feel like searching.