My USB ports stopped noticing new devices when I plugged them in.  I 
tried two different flash drives (one 2.0, one 1.1 -- and I know that 
that shouldn't have worked in a 2.0 hub), both via a hub and directly.  
No messages at all appeared.  I disconnected and reconnected the hub; 
again, there were no messages.  When I disconnected the USB mouse, I 
did get a disconnect message; when I plugged it back in, however, 
nothing happened.

This is -current as of about two days ago, on a Dell Dimension 8400.  
When I rebooted, all was fine.  When the failure occurred, the machine 
had only been up for about two hours.

		--Prof. Steven M. Bellovin, http://www.cs.columbia.edu/~smb