In case this helps someone else, I found a solution to this problem.  By creating a Spring ApplicationListener that listens to the shutdown event, you can signal the jBPM job executor to shut down when the context is stopped.

/**
* A Spring ApplicationListener that will properly shut down
* the jBPM process engine when the Spring context is destroyed.
*
* @author Caine
* Created: Mar 5, 2010 - 4:30:58 PM
*/
@Component
@SuppressWarnings("unchecked")
public class SpringJbpmJobExecutorShutdownHook implements ApplicationListener {
   
    //~~ Static Fields/Initializers ==========================
   
    static final Logger log = Logger.getLogger(SpringJbpmJobExecutorShutdownHook.class);
   
    //~~ Dependencies ========================================
   
    @Resource
    ProcessEngine processEngine;
   
    //~~ Methods =============================================
   
    public void onApplicationEvent(ApplicationEvent event) {
        if (event instanceof ContextClosedEvent ) {
            log.info("Spring ApplicationContext shutting down.  Closing jBPM process engine.");
            processEngine.close();
            log.info("jBPM process engine closed.");
        }
    }
}

More details about the problem, and why it matters can be found on my blog post here:  http://captaincaveman.posterous.com/how-to-shut-down-the-jbpm-43-job-executor-wit