CSS Design: Creating Custom Corners & Borders: A List Apart:

We’ve all heard the rap:

“Sites designed with CSS tend to be boxy and hard-edged. Where are the rounded corners?”

Answer: the rounded corners are right here. In this article, we’ll show how customized borders and corners can be applied to fully fluid and flexible layouts with dynamic
content, using sound and semantically logical markup.

http://www.alistapart.com/articles/customcorners/