Watching Chris Coyiers WordPress video series I saw how rounded corners was used for Mozilla and Safari web browsers using the CSS-styles -moz-border-radius and -webkit-border-radius, but Internet Explorer only showed the square box with no round corners.
I wanted to have rounded corners for my left and right sidebar boxes without having to use images or lots of CSS code. I was reading "The CSS Anthology" by Rachel Andrew looking for ideas and found the section about rounded corners using Nifty Corners Cube (page 371). http://www.html.it/articoli/niftycube/index.html
Very simple to set up in WordPress if you are running your own blog. Download the file, unzip, and install: niftycube.js and niftyCorners.css to your WordPress theme file.
I am using Chris Coyiers .widget style in the CSS: