[CentOS] Root filesystem is suddenly mounted read-only

This is very strange.  Everything was running fine Friday night.   
This morning I walk into work and find that my root file system  
(which is actually the only filesystem besides /boot) is not  
writable.  Yet mount thinks it's mounted read/write:

   # mount
   /dev/mapper/VolGroup00-LogVol00 on / type ext3 (rw)
   [snip]
   # touch foo
   touch: cannot touch `foo': Read-only file system

Anyone else seen this?  This is on a relatively recently fresh CentOS  
4.3 install.

Alfred