I think it's done with JavaScript:

http://acko.net/blog/anti-aliased-nifty-corners

The implementation on drupal.org is done using pure CSS. The idea is  

that you're using those 4 extra divs as hooks to add the 4 background  

images. You specify the position of the background images so they sit  

in each of the 4 corners. There are so many different ways you can do  

it and there are even sites whose sole purpose is to showcase these  

different methods.

http://www.google.com/search?hl=en&q=rounded+corners+css