In case other people are having this problem: A while back (I think when I switched from my old name), I noticed that the threads were not appearing in the order of their last update (Now I'm pretty sure they stay in order of creation). I couldn't figure out why, since I didn't think I'd changed anything, so I just used the "show new only" option that's available. Today I thought I'd go poking around my preferences, and sure enough -- on the Account Preferences page, there's a "Move Updated Topics To Top" checkbox, which was unchecked. Problem solved!