Hi,
The main reason for not exposing the node instance in the work item handler is that "in general" we believe this is not necessary in most use cases. Giving access to the node instance in the handler would in a lot situations lead to handlers that are tightly coupled to the process instance that triggered the service invocation (kinda like how ActionHandlers worked in jBPM3), where they (imho) should be more generic and reusable accross processes.
That said, I can see how access to the node instance id might be useful in some cases (without losing the genericity of the handler). The work item currently has access to the process instance id, so you could just retrieve the process instance from the session and find the node instance that references this work item. We could make an AbstractWorkItemHandler or a utility method that would allow you to do something like that easily.
Wdyt?
Kris