Manik Surtani, Lead of the JBoss Cache team, advises against just wrapping a cluster environment around an application. According to his reasoning in the JBoss Labs Blog&ctrl:window=default.blog.PrjBlogPortletWindowDefaultBlog&project=all&from=1&link=The_Myth_of_Transparent_Clustering#The_Myth_of_Transparent_Clustering, performing in a cluster is something that has to be considered already when building the application.
As much as people would like to think that with modern techniques like AOP, bytecode injection, annotations, along with a healthy dose of ignorance of reality, wishing upon a star and belief in the tooth fairy, clustering can be a truly decoupled aspect that can be applied to anything, they are wrong.