CSS3 supports "border images" which allows you to make very nice looking borders using graphics for the corners and the sides.

However there is currently NO utility/tool/application that allows me to design beautiful CSS3 border images and export them.

Something like the Flex Style Explorer would be a good place to look for inspiration http://examples.adobe.com/flex3/consulting/styleexplorer/Flex3StyleExplorer.html

If only the Flex Style Explorer somehow allowed export of border images.

So there is your new startup idea! Quick, register a domain name and get coding!