You could also use a javascript library to achieve this effect. jQuery has a plugin for it to do Rounded corners. Might want to look into this.
Also, typically -moz-border-radius and border-radius are two CSS elements that work in Mozilla, Opera (you know the lot) except IE. IE is terrible at following rules (much like it's parent company 😛 ), and so you'd have to use an image to physically round the corners.
Just remember that there's power in absolutely positioned elements. You could create 4 small divs that house the "round" corners (which is just a small gif) and use that to your advantage, or use the techniques found in the page above.