Classpath does not seem to take into account the JInternalFrame.setFrameIcon() call. Wether you set it to a real file, to a non-existent one, or even if you don't call it, the internal frame will have the default icon.
Created attachment 11394 [details] Simpe test trying to set an icon for the internalframe
Fixed. http://cvs.savannah.gnu.org/viewcvs/classpath/javax/swing/plaf/metal/MetalInternalFrameTitlePane.java?root=classpath&r1=1.8&r2=1.9