Member Rara Avis
I don't use the Back button much, so hadn't noticed this before you mentioned it. I doubt the behavior is the result of browser setting, but rather is the way Microsoft has decided the browser should work. I suspect they feel they just fixed a bug.
Think about it. When you click on a link into a forum listing, IE recognizes it's a dynamically generated page and ALWAYS asks the server for a fresh copy. Why should the Back button be any different than a link? In theory, the browser should never have bothered to even write a cached page to disk, since it knew the page would change.
Of course, I'm just guessing and my conclusion is based on very limited testing (static versus dynamic pages). But I have this funny feeling the behavior you've described ain't going away.