I would like to try hibernate on an ATG 5.6.1 (jdk 1.3.xxx) clustered environment. Are there things I should take care about ? Is hibernate able to handle its cache on a cluster?

I'm a newbie on this area ... so any input is usefull.

the only "hibernate" cache is a first level cache and is the sesison.
No problem for clustered environment.

Now, you have the choice for the second level cache, you can plug several different caches .
http://www.hibernate.org/hib_docs/refer ... ance-cache

You can also not use any second level cache...

I think for clustered environement, if you really need a second level cache, just use JBoss TreeCache, or think about a commercial cache...