Holy TheServerSide.Com, has cool articles on Clustering :

See http://www.theserverside.com/articles/index.tss and search for
clustering. Also take a look at this :
www.theserverside.com/.../article.tss

If you're looking for clustering tools/tech, take a look at :

Tangosol's Masterpiece : http://www.tangosol.com/coherence.jsp
And GigaSpace' award wining product : http://www.gigaspaces.com/