We have three machines (actually LPARs on big IBM boxes) with two instances of WebSphere and our application on each LPAR. We consider it a cluster of six application servers. Almost all the issues you have with separate machines - sharing state, load balancing, etc - are the same with separate JVMs on the same machine. I guess that would mean the "virtual" machine is pretty "realistic."
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi