On Mac OS X, JButtons render differently based on what you put in them. The buttons on the table header look like rounded Aqua buttons normally and they change shape when you put the up/down arrow icons in them. You tell the button to take a hint and render the same way every time. Hopefully another benefit will be that I get to see more text in the header using the "square" hint.
Here are the available hints from Apple. The Sun Swing docs indicate that if a platform doesn't understand a hint it ignores it, so I think it's safe without having to resort to OS detection.