If you want your JButton to have no decoration across the standard look and feels, there's two things you need to do:
JButton myButton = new JButton(myIcon);
myButton.setBorder(BorderFactory.createEmptyBorder());
myButton.setContentAreaFilled(false);
setBorder prevents the Aqua look and feel from drawing a glassy aqua button
We need setContentAreaFilled to prevent the XP look and feel from drawing a plastic luna button
It's unfortunate that the two look and feels don't use the same API to accomplish the same result.