Twenty Ways

If you have a jailbroken iPhone and find yourself unable to set the passcode lock, you can still activate this feature manually:

• set your passcode on the iPhone under Settings / General / Passcode Lock
  (this stores the passcode, despite the feature not being activated)
• open /private/var/mobile/Library/Preferences/com.apple.springboard.plist on your phone;
  the easiest way is to copy the file locally via SFTP, then edit it in the Property List Editor bundled with Xcode
• add the following key under Root:
  • key = PasswordProtected
  • type = Number
  • value = 1
• restart SpringBoard (relaunch the process, or just restart your phone)

You can also reverse this process if you find yourself unable to access said iPhone.