When you enable iCloud’s Find My Mac feature, the OS X login window will display a Guest User account, even if the Guest User account was never enabled (it’ll say Sharing-only mode if you look at the Accounts system preference pane). Clicking on the Guest User icon from the login window will display the alert “This computer will restart to a secure, Safari-only system for the Guest user”.
When booted into Safari-only mode, your Mac’s location will be tracked using location services, plus the user will only be able to surf the internet using a stripped down version of Safari. Apple’s thought process what that if your Mac was lost or stolen, whoever wound up with it would likely be stuck at the login window (assuming you didn’t turn on automatic login), and the only option they would have to use the Mac would be as a Guest using Safari-only mode.
A couple of things to know about the technology behind Safari-only mode… it actually boots from the Recovery HD partition that is created when you install Lion. If this partition isn’t present on the boot drive, the Safari-only mode option won’t be available. While booted into Safari-only mode, the main boot drive partition is not mounted, and cannot be accessed. When you quit Safari, you are forced to reboot back into the OS X login window. If you select the Guest User account from the login window, and other users are logged into the Mac, their sessions will be terminated without the option to save open documents. The option for Guest User will not show in the login window if it has been configured to display only username and password fields, and entering guest for the username will not work.