Like border-width, you can target the different corners, like so:
border-radius: 0 0 10px 10px;
where the corners are as so:
border-radius: [top-left] [top-right] [bottom-left] [bottom-right];
You can still use the width/height units like you want, like:
border-radius: 0 0 10px 10px / 0 0 30px 30px
Lea Verou had a interesting presentation on border–radius that you might want to check out.