Instances of JTable that use DefaultTableCellRenderer are no longer serializable as of 6u10. The reason is the new field DefaultTableCellHeaderRenderer.emptyIcon, which is set to an instance of the private inner class EmptyIcon. EmptyIcon is not serializable.
This problem was found by a user who returns objects of type JPanel from JMX methods in their application. Invoking the method from JConsole causes the panel to be displayed in a JConsole dialog when the method is invoked. That doesn't work any more if the JPanel contains a JTable.