There's actually a couple of things you can try (in order, least pain to more):
1. Try booting the machine in single-user mode and then edit out (remove) the
encrypted password for root in the /etc/shadow file. This will leave root
with no password.
Example: root:asdfasdfs: .........
|-------------This is the encrypted password.
Continue the boot process into multi-user mode or just reboot. Login as
root and pick a new password (passwd or nispasswd as appropriate).
You may have a problem booting single-user if the PROM is set up for
secure mode. This requires a password to boot single-user.
2. Boot the miniroot from the Solaris 2.3 CD and mount the real root partition
onto, say, /mnt. Do as above by editing /mnt/etc/shadow.
WARNING: This blatant editing of the shadow file may cause problems if you are
running with NIS+ and credentials. The credentials are based on the
current user password.
Hope this helps,
Disclaimer: All that I say and do here is strictly of my own opinion and reflects
nothing, nowhere, nohow.