Two probably related problems.
1. The "New" flags suddenly disappeared from my topic list displays at about 8:45am PDT. I tried logging out, clearing the cache, and coming back, still missing. Posts since 8:45 get "New" flags as usual.
2. If I look at a topic that I've read before, the old posts are collapsed so only the new ones are visible, but if I go back to the topic list and reload the topic, it looks just the same. If it has a "New" flag, it doesn't go away.
Same behavior with IE7 and Firefox 3.0 on Windows XP.