PE is a Tomcat servlet therefore you could apply regular Tomcat load balancing or clustering strategies.
In practice, with modern hardware, we find that clustering is rarely required for performance reasons. You just provision a powerful enough server and stick PE on that with the appropriate amount of subprocess licenses.
However, we have a few customers who require high availability and disaster recovery for their PE instances. In this case they simply have one or more backup PE servers configured and waiting. There is a load balancer in front of the set of production and backup servers. In the case of a problem the load balancer redirects all PE traffic to the backup server(s). Depending on the PE licensing configuration the load balancer in normal use will either direct all traffic to the production servers or will perform simple round-robin balancing.
I have not yet seen a customer implement hot failover of PE servers but I believe that could be achieved through virtualisation technologies.
