Feature geometries have added a new gx:drawOrder elements to place one polygon over another. Likewise the drawOrder element allows overlay to be placed over one another but at present there is no enforcement between order of drawing ground overlays and polygons if both are clampToGround.

Only workaround at present is to change altitudeMode for polygon to be relativeToGround and set its altitude slightly off the ground.

There is an enhancement request submitted for this issue.
http://code.google.com/p/earth-issues/issues/detail?id=198