David,
This is a half-guess, but have you tried adding the hibernate.connection properties you have in your "org.jbpm.task" persistence-unit to the first persistence-unit ("org.jbpm.persistence.jpa").
You're using hbm2ddl.auto = create in both units, so Hibernate should create the tables. This means that it's problem with the connection with the database.
Another possible idea is using the JDBC3 driver for mysql -- but again, that's also just a guess.