Hibernate 3.3 was modularized; i.e. different components were packaged in separate jars, rather than in one huge monolithic jar. Not all of the component jars are shipped with the AS, but you can certainly add ones that you want to use.
The version of Hibernate that ships with AS 5.1.0.GA is 3.3.1. The 3.3.1 ehcache integration jar can be found in our maven repo at http://repository.jboss.org/maven2/org/hibernate/hibernate-ehcache/3.3.1.GA/
Thank you for your answer, it was helpful indeed.