A WebKit demo simulating the behaviour of the Segmented Control in Mac OS X 10.7 (Lion).
CSS3 has been used for all of the visual design and the transitions. Sadly, the demo is not CSS-only. Minimal jQuery has been used to set the state of the Segmented Control.
Update: By request, the demo has been updated to include jQuery animations for browsers that do not support CSS transitions. Of course I also saved the original, clean version for your perusal.