When building Tweet My Gaming (have you seen it advertised in the sidebar on Twitter.com? ;p) we actually decided to use the border-radius CSS property for WebKit and Mozilla-based browsers. In our opinion, the site looks totally acceptable in Internet Explorer and Opera (and other browsers that don’t have similar support), and Safari, Chrome and Firefox get a little extra visual reward.
The usage of the CSS properties is a lot like:
-webkit-border-radius: 4px;
-moz-border-radius: 4px;
With the first targeting Safari, Chrome and other modern WebKit-based browsers, and the second targeting Mozilla-based Gecko browsers.