The article states
To display a dialog in an internal frame, you use one of the showInternalXXXDialog methods in JOptionPane. These internal frame dialogs are not modal.
I think that information is dated. I just put this together and it shows that the internal dialog is indeed modal.
Until the dialog is dismissed the button can't be clicked. And the sysout only appears in the console after the dialog is closed.
Microsoft Windows XP [Version 5.1.2600]
Java(TM) SE Runtime Environment (build 1.6.0_20-b02)