In this article, the authors draw from their practical experience to list and discuss some critical considerations when building J2EE applications so that they can be deployed in a clustered environment without requiring any code changes.

http://www.onjava.com/lpt/a/4993

Someone tested that using Terrcotta is much better than RMI broadcast - http://pojomojo.blogspot.com/2007/01/reducing-bottlenecks-in-jee-stack_14.html