jBPM is opensource, so a list of customers is hard to give. Besides that, many 'commercial' users of jBPM do not want to have their identities disclosed. So unfortunately this is hard to do. If it is really important to you, maybe you should contact JBoss.
There is a list of JBoss JEMS customers (of which jBPM is a part) so maybe some of them use jBPM.