While it's not the 1:1 comparison an initial glance would have you believe, MacRumors has discovered buried within Mac OS X Lion a "Restart to Safari" mode that's very similar to that browser-based OS from Google.
Indeed, on the Mac OS X Lion user lock screen, MacRumors has found an alternative log-in option that allows a user to access their computer using the Safari browser, instead of the usual login. In theory this would allow an unauthorised user to use the computer to browse the internet and nothing else.
So, not nearly the experience a user would expect to see with a Chrome OS netbook or PC, but if you were a business or school looking to use a series of PCs as anonymous internet kiosks, well, there you go. [MacRumors]