Currently OpenSolaris doesn't allow system reboot nor shutdown directly from the login screen (gdm) by default. Since I started using KDE 4.1.4 as my default desktop manager, I've been quite annoyed by this, because this build of KDE doesn't have reboot nor shutdown available from the session either. So when I wanted to save my KDE session, I had to log out and log in again to failsafe terminal to issue a shutdown command.

This issue is tracked by bug 1166. If you want to enable the so called System Menu in your gdm, including reboot and shutdown, here's a couple of simple steps:

$ pfexec perl -i -pe 's:(\[greeter\]):\1\nSystemMenu=true:' /etc/X11/gdm/custom.conf

$ pfexec su -
# echo "gdm::::type=normal;auths=solaris.system.shutdown" >> /etc/user_attr
# exit

And here's the result after gdm is restarted: