I tried to do something similar, and had more trouble than I expected;
  non-0 uid, group operator, with user's shell a script that calls
  shutdown 

It seemed that the script exiting and the login context being cleaned
up right away caused shutdown to fail; I got the 'system going down
immediately' messages, but it didn't actually halt.  I can't check
that particular box now, but I think I kludged around this by putting
in a 'sleep 60' after the shutdown to keep the login session going.

-- 
        Greg Troxel <gdt@ir.bbn.com>