Booting Solaris Intel into single user mode

Press e in when grub shows at boot time.
Edit multiboot line below and add -s to the end press enter
then b for boot

root (hd0,0,a)
kernel /platform/i86pc/multiboot -s
module /platform/i86pc/boot_archive

Or you can edit /boot/grub/menu.lst and add new entries like below copy your original entries in the file and add -s to end of multiboot line.
title Solaris 10 11/06 s10x_u3wos_10 X86
root (hd0,0,a)
kernel /platform/i86pc/multiboot -s
module /platform/i86pc/boot_archive