You might end up with two copies of the same persistence context (EntityManagerFactory), one bootstrapped by JBoss and one by the Spring app context. Also as I recall, JBoss is quite strict with persistence.xml validation so you might end up with validation issues (e.g. explicitly listing entity classes vs classpath scanning). And if you do run into issues, disabling JBoss scanning is easier said than done.
Anyways, do note that if you are using a recent-enough version of Spring (3.1.x+?), you don't need a persistence.xml at all. See http://docs.spring.io/spring/docs/3.2.6.RELEASE/javadoc-api/org/springframework/orm/jpa/LocalContainerEntityManagerFactoryBean.html#setPackagesToScan(java.lang.String...)