try downgrading the hibernate (3) libs in the lib/hibernate directory of jbpm.3 to hibernate 2 libs. and then try to recompile jbpm. not sure that that will work, but you can try.
also try to run the jbpm test suite.
we are interested in your results.
Packages of Hibernate 2 and Hibernate 3 are distinct (net.sf.hibernate vs org.hibernate). Have you tried to put both jars in the classpath?
If you use the second-level cache you must configure each SessionFactory with the correct class.