Entire file system goes read-only

[edwardp-mh2Nk+tgbQM at public.gmane.org]

On one system while it's running, something occurs which causes the 
/entire/ Ubuntu (10.04 LTS) file system to become read-only.  It was 
first discovered two days ago when I tried to clear out the Trash 
folder, when a window appeared indicating files were read-only.  I then 
opened the file manager and /every/ icon had a lock on it.

At this point, the only options were to shut down or reboot.  Upon doing 
so, when the system comes back up, it goes straight into a file system 
check, then when that has completed, the system reboots itself without 
user intervention.

Is there any possible explanation as to what (and why this) is 
happening, or maybe something I can look at?