in version 6, there are:
- dedicated work item handler that does the web service operations
- service task that can work as web service operation invoker
in addition to that jbpm 6 comes with remote interfaces where one of them is REST and the other JMS so you can remotely start processes as well. See documentation for more details about remote interfaces.
I've to use also JBoss ESB, thus I'm using jBPM5.