Transactions and concurrency [hibernate.org]
Topics include: Comparison of Fetching Strategies, Comparison of Mapping Strategies, Using SQL from Hibernate
Includes an example list of all changes required for a particular application.
"Hibernate 3.0 is not source-compatible with Hibernate 2.1, and is not intended as a drop-in replacement for 2.1." More details on the changes between the two versions.