In order to use this you need to install iCloud and enable 'Find my Mac' (this allows you to locate and wipe your Mac should it be stolen). The guest account is then repurposed and will only reboot to the recorvery partition (which is also new in Lion) and only allow guests to open up Safari and connect to WLAN networks. As soon as they quit Safari the computer shuts down.
If you have secured your Mac with a password, logging into the guest account is the only thing thieves can do. iCloud cleverly uses this to goad thieves into going online and broadcasting their position as well as allowing for remote wipes – it even displays instructions for how to connect to WLAN networks when it boots up – while still not allowing them to access any of your files.
This is not supposed to compete with Chrome OS. It doesn't have a password manager and you can't download anything. It also won't remember anything after a reboot.
By the way, you can now also sart up Safari when booting into the recovery partition normally. That's also new and it's just useful but nothing else.
Apples's answer to Chrome OS is the iPad with iOS, not OS X running only Safari. If you don’t understand that then you have understood nothing about Apple.