We're planning to improve our out-of-the-box integration with web service as part of jBPM 5.1. Integration with JBoss ESB is similar to how you integrate with any external service, using domain-specific work items. There is a work item handler that allows you to invoke a service on JBoss ESB already.
Which version of JBoss ESB will include jBPM5 as a service for web service orchestration hasn't been decided yet.
Kris