Rounded corners, are just so beautiful. When you wanted to use them, you could or generate them with JavaScript, or make some pictures in photoshop and then putting them in the background.
This is very labour intensive, especially when you have a lot rounded corners, in multiple divs. Css3 again, can make the life of web designers so much more relaxing! Because rounded corners, will become a standard soon. And of course a few browsers implemented this yet! You just have to use the browser specific prefix. (More about this when you follow the link).