Your StartupService must implement an interface like this:
public interface StartupServiceMBean extends org.jboss.system.Service {...}
For more information:
http://www.jboss.org/index.html?module=bb&op=viewtopic&t=59887
In addition, place the StartupServiceMBean interface in the same package of your class. (Not sure if it is a requirement, since I didn't see such restriction anywhere. But, my service was throwing a "NotCompliantMBeanException: Class does not expose a management interface" at deployment when my interface was in a subpackage).
Thanks Fernando, that fixed a similar problem I was having!
Andrew