I think Sharky2006 has a point. Learning that results you get are cached is quite deflating on its own, but when someone says it in a manner that is derogatory, it can be upsetting.
If a member is posting cached results, chances are it is because they don't know it's cached. If you feel the need to bring it to their attention, I would suggest also providing something constructive, like a mirror they can try, or to clear their cache, etc. etc.