I have not tried it myself yet...
JBoss 5.1.0.GA has this version of hibernate-core in %JBOSS_HOME%\common\lib directory:
Just keep in mind the Hibernate dependencies (like javassit, etc.) when you do the manual upgrade (unless there is a Hibernate/Jboss upgrade script).
Well i tried to upgrade it before but man... how many problems appeared in my way. I gave up, and looked for some how-to. Well, at least now i know about the sl4j. I will give a look again later.