jBPM3 is stable and currently is supported (and will be supported for quite a few more years). For new projects, I would recommend trying to migrate to jBPM5. I've just put out jBPM-5.0-CR1 and we should have a final release in a few weeks time, so I don't think you should be afraid of jBPM5 not being finished yet. It will be productized in 2011.
Regarding migration, we're working on a project that would allow you to transform your existing jPDL processes into BPMN2 processes so they can be migrated to the jBPM5 engine. More info here: http://community.jboss.org/wiki/jBPM5migrationtoolproject