The privacy options available to subscribers give you control over who can view your moblog's content - either anyone, logged in users, only people on your friends list, or full password-protection. The downside to using a setting other than 'anyone' is that your images cannot be highlighted, but it does provide the privacy that some users desire.
Combined the above the 'keep my moblog hidden' option, you can make your moblog invisible to anyone you haven't told about it and given the password to.