I've noticed that when I browse a Vanilla forum in Firefox, using the back button to go from a thread to the index effectivley refreshes it so that the thread is no longer highlighted. However, this doesn't happen in Safari and I have to either click the "Discussions" link or go back and refresh the index to 'unhighlight' threads I've read.
I was just wondering if this is a feature or an accident?
Browsers tend to handle caching in different ways. When using the back button in some browsers, it will just use the last viewed version of the page - other browsers will reload the old page completely. It's all dependant on the browser - not much I can do about it (that I know of).
I think that you can use PHP to send a header that 'forces' a refresh on every page, but not all browsers handle it the same way. So, like mark said, I don't think there's really anything that can be done about it.