So I just checked the jQuery Corner plugin, which I've seen used in the Acquia/TNT/Fusion themes, and its a plugin that adds rounded corners to blocks without using images.
I've just been playing around with it and now I'm thinking where were you when I started putting 28 rounded corners images in a css sprite image and control them with dozens of lines of CSS selectors.
The reason why this type of plugin is so nice with Drupal is that it works perfectly even under these common situations: