The conventional approach for rounding corners on websites has always involved creating images to position creatively via CSS at the top/bottom/etc... of div's or other html elements (table headers or columns and so on).

That convention is painstaking and not the most agile solution to creating rounded corners - for example, when you want the curve radius to be say 5px instead of 10px you've got to jump into photoshop and roll new images etc...

Well, I came across the Rounded Corners module for drupal recently and *love* it!  Install the module, (& I recommend updating the javascript that powers it to ensure x-browser compatibility) then simply create rules from the admin side for particular css rules et voila!

Something to remember; the background color for the corners will be auto-set to the background of whatever layer is directly above the rounded one in the CSS hierarchy for your site.