Extend the webpopup render plugin (see http://nienhueser.de/marble/marble-webpopup.png ) to look more like a popup dialog:

• Use qDrawBorderPixmap to draw a border
• Add a button or icon at the top border which, when clicked, hides the popup
• Draw some indicator (arrow, speech bubble) to the geo position (coordinates)
• Optionally draw an appropriate drop shadow image (use inkscape for that) that is used to render a drop shadow