The normal way to go about creating rounded corners in a fixed layout, is to cut an image into 3 parts: top, center, bottom.
This however won't work for scratch-able layouts, and most techniques covering this aspect, will take advantage of hacks.
I managed to come up with a technique, using nothing less then a bit of extra mark-up. The technique takes heavy advantage of CSS position, and additional divs, to create borders, and corners using images.
Each background-color represent its own part of the image, you will need to cut your image into a total of 9 separate parts for it to work. Check it out: BlueBoden's BorderTechnique