I've previously spent some time exploring the JBoss Community Drools "Business Logic Integration Platform"- and found this blog posting by Mark Proctor that discusses sequential rule engines.
From the OpenL Tablets website, I found the following links to be interesting:
June 4, 2010
OpenL Tablets team is working hard to release a new version of the product. It will be restructured and contain configurable Service Frontend out of the box. The release packaging will also be changed. Several different artifacts will be published at sf.net such as eclipse site, webstudio war, service frontend war and archive with engine jars only. The OpenL Tablets artifacts will be released in public maven repository as well.
The team is going to start moving project documentation to project public wiki. This will take some time and will end up with complete product documentation available at web.
Not all the changes will be done in this new release. The next one will also bring some major changes in rules projects and tools (we’re going to introduce maven based rules projects out of the box, however, do not guarantee to accomplish everything, the release is still being planned).
If you are looking for an open source rule engine, see this link:
These books may also be of some interest: