can't load com.sun.java.swing.plaf.gtk.GTKLookAndFeel

OmegaT 2.3.0.1 crashes at launch on Ubuntu:

pierre@pierre-MacBook:~$ omegat
14784: Info: ===================================================================
14784: Info: OmegaT-2.3.0_1 (Fri Oct 12 11:45:45 CEST 2012) Locale en_US
14784: Info: Java: Sun Microsystems Inc. ver. 1.6.0_24, executed from '/usr/lib/jvm/java-6-openjdk-i386/jre' (LOG_STARTUP_INFO)
Exception in thread "main" java.lang.Error: can't load com.sun.java.swing.plaf.gtk.GTKLookAndFeel
at javax.swing.UIManager.initializeDefaultLAF(UIManager.java:1326)
at javax.swing.UIManager.initialize(UIManager.java:1407)
at javax.swing.UIManager.maybeInitialize(UIManager.java:1395)
at javax.swing.UIManager.getDefaults(UIManager.java:644)
at javax.swing.UIManager.getColor(UIManager.java:686)
at com.vlsolutions.swing.docking.ui.DockingUISettings.<init>(DockingUISettings.java:219)
at com.vlsolutions.swing.docking.ui.DockingUISettings.getInstance(DockingUISettings.java:232)
at com.vlsolutions.swing.docking.DockingDesktop.<clinit>(DockingDesktop.java:139)
at org.omegat.Main.runGUI(Main.java:189)

(This bug has been reported here: https://bugs.launchpad.net/ubuntu/+source/omegat/+bug/1065905\)