Laura, I think this error is caused by mixture of two JPA versions. JBoss AS 7.1.1 uses JPA2 by default, same does jbpm 5.4 installed with jbpm installer. I noticed you still have them deployed there and they are loaded successfully. But in your project you have hibernate 3 which is JPA 1 and the same applies to persistence.xml that you use, it's for version 1.
So I see two options:
1. upgrade your project to use JPA2 and then you don't need to add hibernate jars into your application - this is recommended
2. disable jpa subsystem in the JBoss AS 71.1. so JPA 2 will not get into the way, then you need to remove jbpm applications from the server otherwise many errors will be thrown.
HTH