With Web 2.0, rounded corners have become wildly popular. Unfortunately, as of right now, you need to use transparent images, some fancy CSS hacks and some extraneous HTML to make them work properly.
However, Mozilla (Firefox) and Webkit (Safari, Konqueror) browsers have implemented a property called border-radius, which will apparently be included in the CSS3 spec, allowing you to easily create rounded corners on any bordered HTML element.
At this time, you have to use browser-specific properties for Mozilla and Webkit browsers, as the border-radius property is not yet fully implemented. You can find more information about the -moz-border-radius and -webkit-border-radius implementations on The Art of Web. More info about the proposed CSS3 spec on border-radius can be found on the W3 site.