I'm never going to build this idea, so here it is, for someone else to do.....
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!