It strongly depends on application domain.
Jbpm "understands" mainly two process languages:
jpdl and bpel.
If your application concerns human activities jpdl is the right choice, otherwise if you are thinking about web-service orchestration, bpel is better.
Anyway jbpm has recently changed in its architecture and now has got a central component named process virtual machine (PVM) on top of which you can build a process language specific layer to execute every kind of process definition.
Thanks a lot Paolo, my application concerns human activities, and I'm going to use jpdl :D
By the way, have you any j2ee standard architecture?