It's important to note that these rounded edges won't work in older versions of Internet Explorer, I believe like versions 8 and prior. So keep that in mind and try not to make your layout dependent on the rounded corners. In other words it should still look good with straight corners. Situations like this are sometimes referred to as "degrades gracefully" or "progressive enhancement".
There are some third party tools that are supposed to help older versions of IE support some of the more useful CSS3 features, but they're kind of buggy at the moment: