I noticed that the suid mode was missing and set it with chmod u+s /usr/bin/su. Now the permissions are:
$ ll $(which su) -rwsr-xr-x 1 root root 24120 May 24 2008 /bin/su
And now su -l works for ordinary users. Thank you very much.
I am certain that I have not been changing file modes in /usr/bin, ever. Does anyone have any idea how this change could occur? Where would a file mode change be logged, if at all?
Thanks.