Hi,
For both the intermediate message event as well as the start event you can use the Camel, Mule and eventually the Spring Integration frameworks in combination with Activiti to accomplish this.
For the intermediate message event you can use a receive task that is completed when a message from a certain source (JMS, WS etc) is processed by Camel, Mule or Spring Integration. To be able to do this you need some kind of correlation identifier to lookup the right process instance when such a message arrives.
The same is true for the start event. You can use Camel, Mule or Spring Integration to consume a message from a specific source and start a new process instance from there.
In the future we may start to support the BPMN 2.0 intermediate message event and message start event.
Best regards,