You may also want to look at the Hibernate Core Migration Guide : 3.2.
Specific migration concerns include:
New Second-level Caching SPIs
The old CacheProvider interfaces/impls are still around with bridges to the new APIs, so this is more of a heads-up. Currently only the integration with JBossCache 2.x via the hibernate-jbosscache2 module takes advantage of the new SPIs.
Hibernate has migrated to using slf4j as the logging framework. A significant difference between the two is that slf4j requires that users (at least) select which logging backend they want to use. This is accomplished by defining a dependency on the appropriate logging bridge (slf4j-log4j for example) if using maven or by dropping any needed jars on your classpath. See Steve's module blog entry for details.
Again see Steve's module blog entry for details.