Take a look at Tangosol's Coherence
clustered caching product. Coherence has an HTTP Replication Module for Servlet 2.3 application servers which does not limit the size of the session that is replicated across the cluster nodes.
Further, if you are interested in persisting the information stored in the replicated clustered cache Coherence provides a "write-through" or "write-behind" feature allowing you to do so.
: Easily share live data across a cluster!