I've tried this on Ubuntu Firefox, Ubuntu Chrome, Windows Firefox, and Windows Chrome, and so far, only Windows Firefox doesn't have this problem:
When I log into the forum, go to a forum topic, and try to submit a reply, it says that I'm not logged in. The header says I'm logged in, and it's right after I load the page, so it shouldn't have timed out yet.
I'm sure I'm not the only person using a browswer other than Win/FF, so what gives?