This seems to have done it:

    public HtmlDropDownMenu getNodeMenu()

    {
        HtmlDropDownMenu menu = new HtmlDropDownMenu();
        menu.setValue( "Node Select" );

        HtmlMenuItem menuItem = new HtmlMenuItem();
        // TODO programmatically pass from getNodes into a String[] rather than an ArrayList of SelectItems
        String subOption = "myitem";            
        menuItem.setValue( subOption );

        Application app = FacesContext.getCurrentInstance().getApplication();
        MethodExpression methodExpression = app.getExpressionFactory().createMethodExpression(
                 FacesContext.getCurrentInstance().getELContext(), 
                 "#{BackingBean.onItemClick}", 
                 null, 
                 new Class[] { ActionEvent.class });
        menuItem.setActionExpression(methodExpression);

        menu.getChildren().add( menuItem );
        return( menu );
    }

    public void onItemClick( ActionEvent event )
    {
        Object obj = event.getSource();

        if( obj instanceof HtmlMenuItem )
        {
            HtmlMenuItem item =    (HtmlMenuItem)obj;
            if( item != null )
            {
                lastItem = item.getValue().toString();
                logger.info("lastItem selected is " + lastItem);
            }
        }
    }

oh, and in your jsp:

                <h:form>
                    <rich:dropDownMenu binding="#{PrismBacking.nodeMenu}">
                    </rich:dropDownMenu>
                </h:form>