Using instructions provided by The Unofficial Apple Weblog back in 2008, I was able to boot up in Single-User Mode and use Unix commands to remove the file signifying that Apple Setup was complete.
Note that the final command they list ( shutdown /h now) does not work. Just type the word exit in the command line instead.
Upon reboot, I was able to step through the set-up assistant portion of the OS X Lion installation process.
I set-up a new admin user making sure to use a unique full name, username, and password (not the same as the profile that's missing and needs to be restored). After finishing the set-up process, I was able to login as an admin level user with the newly created profile.
Once you are back in the system as an admin, click on the System Preferences icon in the Dock. When the System Prefernces pane opens, select "Accounts" under the "System" section. Then create a new user profile, but this time input the exact full name, username, and password of the missing admin profile you want to restore. Be sure to assign it an admin level status. A message will pop up telling you that a directory with that username already exists. Click the option that states something like "use existing directory." This will attach the new profile you're creating to the orphaned user directory. Be patient. The pinwheel will spin for up to 5 minutes as the Mac creates the new profile and links it to the existing directory.
When it's done, you will see the new profile show up in the user list. You may logout and login with your restored profile.
I decided to keep the temporary admin profile I created as a backup login in case something ever goes awry with my primary profile. Perhaps there's a drawback to that decision, but it seemed like a good idea.
