On with solaris level are you working on.

Since version 2.6 solaris ask you for enter the root password when you boot single user, so this will not help you.

the only way, as far i see is to boot from cdrom.
stop the installation fase and mount the root-file-system.
make a copy of the password file, reset the password field in the password file for user root.

reboot your system, set the password again.