OT: remount a dir r/o

[list user]

Hi all,

Sorry for the OT post but I've been experimenting with exporting 
filesystems, xen, and lvm and would like to change the mount permissions 
on a subtree of a filesystem.

I've tried all sorts of permutations of the "mount" command and succeed 
only in getting "help".

I would like to remount "/usr" to be read only.  Anybody willing to take 
a stab at this?

Thanks for any help!