Sun has a cool infodoc that describes adding a single user mode entry to the X64 Solaris 10 grub menu. This is super simple to do, and the following should more or less work (you need to adjust hdX to match your primary or secondary boot device):
$ echo " title Solaris 10 1/06 Single User Boot root (hd0,2,a) kernel /platform/i86pc/multiboot -s module /platform/i86pc/boot_archive " >> /boot/grub/menu.lst
I plan to add this to my crib notes. Jeah!