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.
This is a nice feature for businesses that utilize web kiosks or users who want to share access to their computer.
It's unclear why Apple didn't mention it earlier. Perhaps, the company is still looking to surprise with a few new Lion features at release.