To be really flexible in my CMS I decided to use a persistence unit with one name which will be configured in the final webapplication. This means I have a persistence unit “cms” in my business-components in my cms.jar and this unit is configured via persistence.xml in the including web-application, which uses my “cms.jar”. So every webapplication has its own database and I can reuse my businesslogic for the cms (retrieve content, or images and so on).
The problem now is that I have some problems with deploying two applications on JBoss 5.1.0.GA which includes my cms, because there is a bug in this application-server. I hope this will be fixed in the one of the next versions.