See Dmitri Maximovich's Blog. With very interesting articles about:

• CMP entity Ejbs clustering behaviour regarding 'Optimistic concurrency' strategy
• Weblogic server v9 new entity Ejb clustering features

If entity Ejbs 2.1 are a “nightmare” to develop and test compared to transparent persistence frameworks of POJOs like Hibernate, some containers like BEA's one offer nice features to build up a R/W distributed cache (of persistent data).

But it's also true that some distributed cache products can also plug into hibernate… see Hibernate's doc and Tangosol