Sorry - don't know much about Intel-based Solaris, but I assume you assigned the root password on installation? If that's correct, are you sure you're typing the password in the correct case?
Otherwise, can you boot cdrom -s from the installation CD and mount the root partition on the cd to change the root password that way (edit /etc/shadow to delete the encrypted password). Then unmount the root partition and reboot normally and you should be able to login without a password. Always reinstate the password after following this procedure.
As I say, this is the way you would do it in unix, whether the same applies in Intel, others may be able to advise. Hope this helps.