To reproduce:
* Run SwingSet2 on a RedHat 9.0 machine
* Switch to the GTK look & feel

The following exception is thrown:

java.lang.ClassCastException: java.lang.Long
        at javax.swing.JSplitPane.setUIProperty(JSplitPane.java:1056)
        at javax.swing.LookAndFeel.installProperty(LookAndFeel.java:144)
        at javax.swing.plaf.synth.SynthSplitPaneUI.updateStyle(SynthSplitPaneUI.java:114)
        at javax.swing.plaf.synth.SynthSplitPaneUI.installDefaults(SynthSplitPaneUI.java:62)
        at javax.swing.plaf.basic.BasicSplitPaneUI.installUI(BasicSplitPaneUI.java:288)
        at javax.swing.JComponent.setUI(JComponent.java:601)
        at javax.swing.JSplitPane.setUI(JSplitPane.java:354)
        at javax.swing.JSplitPane.updateUI(JSplitPane.java:382)
        at javax.swing.SwingUtilities.updateComponentTreeUI0(SwingUtilities.java:1150)
        at javax.swing.SwingUtilities.updateComponentTreeUI0(SwingUtilities.java:1161)
        at javax.swing.SwingUtilities.updateComponentTreeUI(SwingUtilities.java:1142)
        at SwingSet2$6.run(SwingSet2.java:981)
        at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:188)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:459)
        at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchThread.java:214)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:163)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:157)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:149)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:110)

I've only seen this on RH9.0, not 7.2, 7.2-64, or AS2.1. It may be a GNOME 2.0 thing, and possibly reproducible on Solaris as well. It may be necessary to wait for b19, as I can't reproduce this in b18.