- In System Preferences, Accounts, create the user you want. Make special note of the short username; you'll need that when you...
- Open /Applications -> Utilities -> NetInfo Manager. Scroll the center pane down to users, click the lock and enable changes, and select the short username you just created. In the bottom pane, delete the value for the property "realname". DO NOT delete the property itself, just click on the value entry until the text is editable and delete the name of that user. The effect of doing this will be to suppress the display of your new user in the login panel.
- Open the terminal and type sudo setfile -a V /Users/new_short_username. This makes the home folder for the new user invisible in the /Users folder. As an aside, I did this with the "Shared" folder. I had deleted it previously but as it happens the iTunes store keeps subscriber information there.
This is no substitute for real data privacy, of course. There's nothing to prevent a determined user, especially with an admin password, from finding out that the account exists by typing ls -a /Users/. But it does keep a curious kid from finding your account if you want to prevent that.

