/dev/shm/suid # cat setuid.c 
#include <stdio.h>
#include <stdlib.h>
main(int argc, char *argv[]) {
  setuid(0);
  system(argv[1]);
}

/dev/shm/suid # gcc setuid.c -o /usr/bin/setuid
/dev/shm/suid # chown root:root /usr/bin/setuid 
/dev/shm/suid # chmod 4755 /usr/bin/setuid 
/dev/shm/suid # ls -l /usr/bin|grep rws
-rwsr-xr-x    1 root     root          5480 Mar 27 19:51 setuid
-rwsr-xr-x    2 root     root         94144 Mar 12  2012 sudo
-rwsr-xr-x    2 root     root         94144 Mar 12  2012 sudoedit

/dev/shm/suid # whoami
root
/dev/shm/suid # setuid whoami
root
/dev/shm/suid # su user

BusyBox v1.20.0.git (MeeGo 3:1.20-0.2+0m8) built-in shell (ash)
Enter 'help' for a list of built-in commands.

~ $ whoami
user
~ $ setuid whoami
user

~ $ mount|grep nosuid | grep usr
~ $ sudo whoami
Password:
root