yeah that's the behaviour for jBPM3, if you are using jBPM4 it's a different story, the user starting the process is stored in the history, see this thread: http://community.jboss.org/thread/154420
furthermore as far as i understand the concept of a swimlane: It's job is to provide a kind of "process role" in order to assign the same user(s) to a number of tasks, being able to override the assigned user like you describe is not a problem in my opinion, you application code should handle this.