I recently managed to find the time to install Solaris 10 on my Ultra2. Since I've now moved away from assigning static ip's on my network (except for a few essential hosts) I choose the DHCP option when running the installer. Interestingly the installer never asked for a hostname during setup and sure enough when I first booted my Ultra2 it was now known as 'unknown' and of course no hostname was registered in DNS. To fix this I created the file /etc/nodename which contained just the hostname for the system and /etc/hostname.hme0 which contains one line:
The suffix in the above file name refers to the interface name.
A reboot later my Ultra2 was successfully registering its hostname in DNS.