The result
Here is a sample screenshot what the result could look like

This method is used in Red Innovation skin, available at plone.org product area. I recommend you to download this skin and recycle its TAL and CSS code, so you don't need to reinvent the wheel. Don't expect the process be so straightforward as explained here. E.g. deprecated.css always overrides some CSS styles which forced me to create new CSS styles for portlet content, instead of just redeclaring old styles.
The method is in live usage at www.redinnovation.com.
Markus Proske's MagicSkin product does also corner rounding using images. In his approach, a common portlet template macro is not used.
