Apple snuck a new feature in the Lion beta called "Restart to Safari," which gives Lion a browser user interface -- kinda like Google Chrome OS.
"On Mac OS X Lion's user lock screen, you are given the option to "Restart to Safari" rather than logging in. This allows you to boot the computer into just the web browser and nothing else. This browser only mode allows unauthorized users to simply browse the web through Safari rather than having access to any personal files or other applications on the machine. Of course with Lion's new auto-save and application restore feature, returning users will find themselves back at the same place they were before the restart."
This feature seems to be part of the "Find My Mac" feature set which allows users to get access to the internet. Though the premise appears to be to give your Mac a chance to phone home, the feature seems very useful for day to day use.
Mac OS X Lion will become available for $29.99 on the Mac App Store in July.