We're basing a medium sized application on jBPM, and had been using 3.0. Having modified it to suit our purposes a bit, we noted that many of the modifications we had made were covered in 3.1, so we went at an upgrade.
not a terribly difficult process, but there were some surprises, some great things, and some frustrations.
I wrote up a blog entry on the experience this morning. If you're on 3.0 and looking at 3.1, it's worth the jump.
more info at: http://blogs.sourceallies.com/roller/page/joedeveloper?entry=jbpm_upgrade_3_0_to1