Mike Gerdts has a nice blog entry on how to Boot OpenSolaris into Single User Mode. Unfortunately, it appears that his image links are broken, so the screen snapshots that he refers to in the text are not visible (at least, not when using Firefox 2 on OpenSolaris 2008.05).
So I have taken his instructions, made a few edits, and provided screen snapshots of my own.
When you first start the machine, you will see a GRUB menu. The exact contents will vary, but there should be at least one entry for OpenSolaris. As an example, this is from one of my machines:
With the entry for OpenSolaris highlighted, press the 'e' key and that should take you to:
In the example shown above, I used the down-arrow key on the keyboard to highlight the second line, since that is the one I want to edit. I press the 'e' key again to edit that line, which takes me to:
Add -s to the end of that line:
And then press the Enter key to go back to the list of commands:
Press the 'b' key and the system will begin to boot. You will eventually be prompted for the root password:
After you put in the root password, a shell prompt will be displayed and you're ready to do whatever system administration is needed that requires single user mode.
When you no longer need single user mode, you can have the boot process pick up where it left off. As the documentation points out, all you have to do is press Ctrl-D.