In the Hibernate2.0 docs you find :

in one palce
4.1.3. Provide an identifier property (optional)
The identifier property is optional. You can leave it off and let Hibernate keep track of object identifiers internally.

and another
1.3. Mapping the cat
Every persistent class has to have an identifer attribute...

I am confused, is there a contradiction concerning identifier or am I missing somthing?

There is, I'm currently cleaning the documentation. The property is optional, but recommended if you want to use detached objects (objects that have been loaded in one Session and reattached to a second Session with optimistic concurrency).