I was trying the test case from bug 6369498 to see if it was reproducible.
I could not reproduce that problem but I did see the following exception
on mustang b65 and 72 when remote displaying from Linux or Solaris 10 SPARC back
to Solaris 10 SPARC. The test case is in bug 6369498

% /java/re/jdk/1.6.0/promoted/latest/binaries/solaris-sparc/bin/java AppMain
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at com.sun.java.swing.plaf.gtk.GTKLookAndFeel.initSystemColorDefaults(GTKLookAndFeel.java:1223)
        at com.sun.java.swing.plaf.gtk.GTKLookAndFeel.loadStyles(GTKLookAndFeel.java:1474)
        at com.sun.java.swing.plaf.gtk.GTKLookAndFeel.access$000(GTKLookAndFeel.java:37)
        at com.sun.java.swing.plaf.gtk.GTKLookAndFeel$WeakPCL$1.run(GTKLookAndFeel.java:1414)
        at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:597)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168) at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160) at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)


It seems possible it is related to 6369498, since just as reported by the submitter
of that bug, it can be avoided by calling
UIManager.getInstalledLookAndFeels
before calling
 UIManager.setLookAndFeel("com.sun.java.swing.plaf.gtk.GTKLookAndFeel");