set these prom variables appropriately:

     AutoLoad          Controls whether the system boots automatically on
                       reset or power cycle.  Can be set to Yes or No.
                       Previously, this function was controlled by setting
                       bootmode to c or m.  This variable is overridden by the
                       rebound variable and the reboot_on_panic kernel tunable
                       parameter.  This variable is stored in nonvolatile RAM.

     autopower         Specifies whether a setting of y allows a system with
                       software power control to automatically power back on
                       after an AC power failure.  The default setting of n
                       requires the power switch to be pressed to restart the
                       system.  This variable is stored in nonvolatile RAM.

     rebound           Specifies that the system should automatically reboot
                       after a kernel panic if this variable is set to y.  The
                       variable interacts with the AutoLoad variable and the
                       reboot_on_panic kernel tunable parameter.