Demian,
I was doing the same as your example except my itemDefinition had a structureRef, some examples define it and some don't.
Then I copied my project and started removing tasks to make it simple. After I deleted the Human Task preceding my Service Task the parameter was correctly passed to the service method.
It looks like I have to propagate the POJO past the Human Task. What am I supposed to do? I'm using a copy of Kris TestWorkItemHandler ( it's still in jUnit).