I've run into an issue when using Seam and JBPM where the taskInstance is null on newly assigned tasks. This is only affecting events and email notifications when notify="yes" in the process definition. I believe this to be a jBPM issue, but they asked me to post over here as well and see if anybody had any thoughts or feedback.
http://jboss.org/index.html?module=bb&op=viewtopic&t=109507
Thanks,