Just a late note on this. The forum logs you out after 15min of inactivity. If you have the 'keep me logged in' check box set you don't notice as it logs you straight back in without telling you it had logged you out... however this has the effect of resetting the unread posts as these are cleared when you log out.
I can often take more than 15min reading a long thread or composing a long message, so find Today's Posts better.