When viewing the Inbox in Gmail: When old conversations receive new messages, the name of the sender appears in the list of senders in bold, but the conversation stays locked at the time that the first message was received, rather than updating and causing the thread to bubble up to the top of the page. Curiously, the incorrect last-message time and the resulting sorting problem are not present on other views ("All Mail" or one of the label views).

I've reported this bug to the Gmail team. This seems like another kind of Gmail time-stamp issue, different from the one previously reported:
http://blogoscoped.com/forum/125167.html

Weird. It sounds similar to a muted thread:

http://mail.google.com/support/bin/answer.py?answer=47787

Except when a thread is muted, it doesn't appear in the inbox, so I guess it can't be that...