Take a look at the acpi(4) manpage. There are two tunables that might be of use: hw.acpi.disable_on_reboot and hw.acpi.handle_reboot. You can set those from a shell before rebooting. If one of them fixes it, you can edit /etc/sysctl.conf to automatically set it at boot time. Also, just to make sure this isn't a regression in 11, you might want to download the appropriate (CD or memstick) image for 10.3 and booting from that. You don't need to do another install, just select the Live CD option from the menu to get a shell, and then just do a
# shutdown -r now and see if that can successfully reboot your system. You might also try the same thing with 11 to see if it is triggered by something during the installation process or always happens on 11. sysctl hw.acpi.disable_on_reboot=1 reboot and shutdown -r now and
hw.acpi.disable_on_reboot=1 reboot and shutdown -r now , in both cases no luck won't reboot.