Weblogtools Collection has a very nice article: Scaling WordPress Part 1: Using MySQL Replication and HyperDB written by Keith Dsouza, for anyone who has or aspires to having lots of WordPress Traffic.
Keith mentions WordPress Super Cache for sites handling heavy traffic. For sites with more traffic the post covers:
What Is MySQL Replication or Master-Slave Setup?
How Does MySQL Replication Help?
How Can You Replicate MySQL Data In A Master-Slave Environment?
How do I use a MySQL Master-Slave Setup with WordPress?
Article at : http://weblogtoolscollection.com/archives/2010/03/27/scaling-wordpress-part-1-using-mysql-replication-and-hyperdb/